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From 7c25e4dc9a5a8d07f2c59fd2160bb22c774d1d7a Mon Sep 17 00:00:00 2001
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Message-Id: <7c25e4dc9a5a8d07f2c59fd2160bb22c774d1d7a.1387382496.git.minovotn@redhat.com>
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In-Reply-To: <c5386144fbf09f628148101bc674e2421cdd16e3.1387382496.git.minovotn@redhat.com>
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References: <c5386144fbf09f628148101bc674e2421cdd16e3.1387382496.git.minovotn@redhat.com>
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From: Nigel Croxon <ncroxon@redhat.com>
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Date: Thu, 14 Nov 2013 22:52:40 +0100
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Subject: [PATCH 04/46] rdma: add documentation
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RH-Author: Nigel Croxon <ncroxon@redhat.com>
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Message-id: <1384469598-13137-5-git-send-email-ncroxon@redhat.com>
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Patchwork-id: 55688
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O-Subject: [RHEL7.0 PATCH 04/42] rdma: add documentation
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Bugzilla: 1011720
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RH-Acked-by: Orit Wasserman <owasserm@redhat.com>
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RH-Acked-by: Amit Shah <amit.shah@redhat.com>
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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Bugzilla: 1011720
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https://bugzilla.redhat.com/show_bug.cgi?id=1011720
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>From commit ID:
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commit f4abc9d621823b14a6cd508c66c1ecb21f96349e
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Author: Michael R. Hines <mrhines@us.ibm.com>
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Date:   Tue Jun 25 21:35:27 2013 -0400
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    rdma: add documentation
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    docs/rdma.txt contains full documentation,
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    wiki links, github url and contact information.
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    Reviewed-by: Juan Quintela <quintela@redhat.com>
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    Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
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    Reviewed-by: Chegu Vinod <chegu_vinod@hp.com>
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    Tested-by: Chegu Vinod <chegu_vinod@hp.com>
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    Tested-by: Michael R. Hines <mrhines@us.ibm.com>
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    Signed-off-by: Michael R. Hines <mrhines@us.ibm.com>
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    Signed-off-by: Juan Quintela <quintela@redhat.com>
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---
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 docs/rdma.txt |  415 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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 1 files changed, 415 insertions(+), 0 deletions(-)
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 create mode 100644 docs/rdma.txt
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Signed-off-by: Michal Novotny <minovotn@redhat.com>
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---
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 docs/rdma.txt | 415 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
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 1 file changed, 415 insertions(+)
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 create mode 100644 docs/rdma.txt
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diff --git a/docs/rdma.txt b/docs/rdma.txt
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new file mode 100644
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index 0000000..45a4b1d
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--- /dev/null
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+++ b/docs/rdma.txt
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@@ -0,0 +1,415 @@
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+(RDMA: Remote Direct Memory Access)
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+RDMA Live Migration Specification, Version # 1
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+==============================================
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+Wiki: http://wiki.qemu.org/Features/RDMALiveMigration
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+Github: git@github.com:hinesmr/qemu.git, 'rdma' branch
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+
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+Copyright (C) 2013 Michael R. Hines <mrhines@us.ibm.com>
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+
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+An *exhaustive* paper (2010) shows additional performance details
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+linked on the QEMU wiki above.
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+
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+Contents:
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+=========
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+* Introduction
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+* Before running
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+* Running
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+* Performance
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+* RDMA Migration Protocol Description
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+* Versioning and Capabilities
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+* QEMUFileRDMA Interface
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+* Migration of pc.ram
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+* Error handling
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+* TODO
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+
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+Introduction:
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+=============
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+
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+RDMA helps make your migration more deterministic under heavy load because
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+of the significantly lower latency and higher throughput over TCP/IP. This is
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+because the RDMA I/O architecture reduces the number of interrupts and
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+data copies by bypassing the host networking stack. In particular, a TCP-based
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+migration, under certain types of memory-bound workloads, may take a more
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+unpredicatable amount of time to complete the migration if the amount of
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+memory tracked during each live migration iteration round cannot keep pace
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+with the rate of dirty memory produced by the workload.
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+
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+RDMA currently comes in two flavors: both Ethernet based (RoCE, or RDMA
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+over Convered Ethernet) as well as Infiniband-based. This implementation of
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+migration using RDMA is capable of using both technologies because of
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+the use of the OpenFabrics OFED software stack that abstracts out the
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+programming model irrespective of the underlying hardware.
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+
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+Refer to openfabrics.org or your respective RDMA hardware vendor for
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+an understanding on how to verify that you have the OFED software stack
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+installed in your environment. You should be able to successfully link
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+against the "librdmacm" and "libibverbs" libraries and development headers
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+for a working build of QEMU to run successfully using RDMA Migration.
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+
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+BEFORE RUNNING:
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+===============
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+
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+Use of RDMA during migration requires pinning and registering memory
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+with the hardware. This means that memory must be physically resident
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+before the hardware can transmit that memory to another machine.
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+If this is not acceptable for your application or product, then the use
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+of RDMA migration may in fact be harmful to co-located VMs or other
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+software on the machine if there is not sufficient memory available to
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+relocate the entire footprint of the virtual machine. If so, then the
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+use of RDMA is discouraged and it is recommended to use standard TCP migration.
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+
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+Experimental: Next, decide if you want dynamic page registration.
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+For example, if you have an 8GB RAM virtual machine, but only 1GB
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+is in active use, then enabling this feature will cause all 8GB to
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+be pinned and resident in memory. This feature mostly affects the
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+bulk-phase round of the migration and can be enabled for extremely
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+high-performance RDMA hardware using the following command:
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+
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+QEMU Monitor Command:
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+$ migrate_set_capability x-rdma-pin-all on # disabled by default
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+
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+Performing this action will cause all 8GB to be pinned, so if that's
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+not what you want, then please ignore this step altogether.
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+
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+On the other hand, this will also significantly speed up the bulk round
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+of the migration, which can greatly reduce the "total" time of your migration.
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+Example performance of this using an idle VM in the previous example
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+can be found in the "Performance" section.
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+
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+Note: for very large virtual machines (hundreds of GBs), pinning all
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+*all* of the memory of your virtual machine in the kernel is very expensive
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+may extend the initial bulk iteration time by many seconds,
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+and thus extending the total migration time. However, this will not
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+affect the determinism or predictability of your migration you will
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+still gain from the benefits of advanced pinning with RDMA.
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+
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+RUNNING:
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+========
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+
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+First, set the migration speed to match your hardware's capabilities:
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+
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+QEMU Monitor Command:
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+$ migrate_set_speed 40g # or whatever is the MAX of your RDMA device
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+
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+Next, on the destination machine, add the following to the QEMU command line:
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+
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+qemu ..... -incoming x-rdma:host:port
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+
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+Finally, perform the actual migration on the source machine:
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+
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+QEMU Monitor Command:
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+$ migrate -d x-rdma:host:port
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+
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+PERFORMANCE
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+===========
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+
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+Here is a brief summary of total migration time and downtime using RDMA:
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+Using a 40gbps infiniband link performing a worst-case stress test,
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+using an 8GB RAM virtual machine:
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+
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+Using the following command:
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+$ apt-get install stress
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+$ stress --vm-bytes 7500M --vm 1 --vm-keep
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+
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+1. Migration throughput: 26 gigabits/second.
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+2. Downtime (stop time) varies between 15 and 100 milliseconds.
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+
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+EFFECTS of memory registration on bulk phase round:
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+
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+For example, in the same 8GB RAM example with all 8GB of memory in
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+active use and the VM itself is completely idle using the same 40 gbps
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+infiniband link:
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+
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+1. x-rdma-pin-all disabled total time: approximately 7.5 seconds @ 9.5 Gbps
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+2. x-rdma-pin-all enabled total time: approximately 4 seconds @ 26 Gbps
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+
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+These numbers would of course scale up to whatever size virtual machine
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+you have to migrate using RDMA.
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+
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+Enabling this feature does *not* have any measurable affect on
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+migration *downtime*. This is because, without this feature, all of the
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+memory will have already been registered already in advance during
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+the bulk round and does not need to be re-registered during the successive
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+iteration rounds.
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+
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+RDMA Protocol Description:
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+==========================
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+
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+Migration with RDMA is separated into two parts:
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+
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+1. The transmission of the pages using RDMA
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+2. Everything else (a control channel is introduced)
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+
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+"Everything else" is transmitted using a formal
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+protocol now, consisting of infiniband SEND messages.
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+
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+An infiniband SEND message is the standard ibverbs
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+message used by applications of infiniband hardware.
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+The only difference between a SEND message and an RDMA
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+message is that SEND messages cause notifications
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+to be posted to the completion queue (CQ) on the
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+infiniband receiver side, whereas RDMA messages (used
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+for pc.ram) do not (to behave like an actual DMA).
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+
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+Messages in infiniband require two things:
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+
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+1. registration of the memory that will be transmitted
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+2. (SEND only) work requests to be posted on both
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+   sides of the network before the actual transmission
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+   can occur.
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+
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+RDMA messages are much easier to deal with. Once the memory
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+on the receiver side is registered and pinned, we're
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+basically done. All that is required is for the sender
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+side to start dumping bytes onto the link.
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+
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+(Memory is not released from pinning until the migration
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+completes, given that RDMA migrations are very fast.)
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+
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+SEND messages require more coordination because the
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+receiver must have reserved space (using a receive
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+work request) on the receive queue (RQ) before QEMUFileRDMA
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+can start using them to carry all the bytes as
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+a control transport for migration of device state.
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+
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+To begin the migration, the initial connection setup is
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+as follows (migration-rdma.c):
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+
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+1. Receiver and Sender are started (command line or libvirt):
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+2. Both sides post two RQ work requests
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+3. Receiver does listen()
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+4. Sender does connect()
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+5. Receiver accept()
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+6. Check versioning and capabilities (described later)
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+
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+At this point, we define a control channel on top of SEND messages
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+which is described by a formal protocol. Each SEND message has a
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+header portion and a data portion (but together are transmitted
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+as a single SEND message).
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+
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+Header:
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+    * Length  (of the data portion, uint32, network byte order)
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+    * Type    (what command to perform, uint32, network byte order)
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+    * Repeat  (Number of commands in data portion, same type only)
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+
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+The 'Repeat' field is here to support future multiple page registrations
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+in a single message without any need to change the protocol itself
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+so that the protocol is compatible against multiple versions of QEMU.
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+Version #1 requires that all server implementations of the protocol must
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+check this field and register all requests found in the array of commands located
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+in the data portion and return an equal number of results in the response.
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+The maximum number of repeats is hard-coded to 4096. This is a conservative
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+limit based on the maximum size of a SEND message along with emperical
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+observations on the maximum future benefit of simultaneous page registrations.
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+
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+The 'type' field has 10 different command values:
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+    1. Unused
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+    2. Error              (sent to the source during bad things)
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+    3. Ready              (control-channel is available)
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+    4. QEMU File          (for sending non-live device state)
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+    5. RAM Blocks request (used right after connection setup)
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+    6. RAM Blocks result  (used right after connection setup)
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+    7. Compress page      (zap zero page and skip registration)
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+    8. Register request   (dynamic chunk registration)
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+    9. Register result    ('rkey' to be used by sender)
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+    10. Register finished  (registration for current iteration finished)
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+
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+A single control message, as hinted above, can contain within the data
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+portion an array of many commands of the same type. If there is more than
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+one command, then the 'repeat' field will be greater than 1.
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+
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+After connection setup, message 5 & 6 are used to exchange ram block
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+information and optionally pin all the memory if requested by the user.
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+
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+After ram block exchange is completed, we have two protocol-level
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+functions, responsible for communicating control-channel commands
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+using the above list of values:
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+
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+Logically:
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+
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+qemu_rdma_exchange_recv(header, expected command type)
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+
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+1. We transmit a READY command to let the sender know that
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+   we are *ready* to receive some data bytes on the control channel.
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+2. Before attempting to receive the expected command, we post another
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+   RQ work request to replace the one we just used up.
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+3. Block on a CQ event channel and wait for the SEND to arrive.
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+4. When the send arrives, librdmacm will unblock us.
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+5. Verify that the command-type and version received matches the one we expected.
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+
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+qemu_rdma_exchange_send(header, data, optional response header & data):
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+
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+1. Block on the CQ event channel waiting for a READY command
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+   from the receiver to tell us that the receiver
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+   is *ready* for us to transmit some new bytes.
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+2. Optionally: if we are expecting a response from the command
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+   (that we have no yet transmitted), let's post an RQ
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+   work request to receive that data a few moments later.
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+3. When the READY arrives, librdmacm will
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+   unblock us and we immediately post a RQ work request
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+   to replace the one we just used up.
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+4. Now, we can actually post the work request to SEND
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+   the requested command type of the header we were asked for.
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+5. Optionally, if we are expecting a response (as before),
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+   we block again and wait for that response using the additional
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+   work request we previously posted. (This is used to carry
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+   'Register result' commands #6 back to the sender which
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+   hold the rkey need to perform RDMA. Note that the virtual address
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+   corresponding to this rkey was already exchanged at the beginning
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+   of the connection (described below).
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+
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+All of the remaining command types (not including 'ready')
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+described above all use the aformentioned two functions to do the hard work:
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+
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+1. After connection setup, RAMBlock information is exchanged using
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+   this protocol before the actual migration begins. This information includes
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+   a description of each RAMBlock on the server side as well as the virtual addresses
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+   and lengths of each RAMBlock. This is used by the client to determine the
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+   start and stop locations of chunks and how to register them dynamically
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+   before performing the RDMA operations.
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+2. During runtime, once a 'chunk' becomes full of pages ready to
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+   be sent with RDMA, the registration commands are used to ask the
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+   other side to register the memory for this chunk and respond
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+   with the result (rkey) of the registration.
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+3. Also, the QEMUFile interfaces also call these functions (described below)
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+   when transmitting non-live state, such as devices or to send
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+   its own protocol information during the migration process.
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+4. Finally, zero pages are only checked if a page has not yet been registered
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+   using chunk registration (or not checked at all and unconditionally
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+   written if chunk registration is disabled. This is accomplished using
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+   the "Compress" command listed above. If the page *has* been registered
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+   then we check the entire chunk for zero. Only if the entire chunk is
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+   zero, then we send a compress command to zap the page on the other side.
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+
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+Versioning and Capabilities
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+===========================
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+Current version of the protocol is version #1.
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+
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+The same version applies to both for protocol traffic and capabilities
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+negotiation. (i.e. There is only one version number that is referred to
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+by all communication).
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+
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+librdmacm provides the user with a 'private data' area to be exchanged
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+at connection-setup time before any infiniband traffic is generated.
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+
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+Header:
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+    * Version (protocol version validated before send/recv occurs), uint32, network byte order
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+    * Flags   (bitwise OR of each capability), uint32, network byte order
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+
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+There is no data portion of this header right now, so there is
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+no length field. The maximum size of the 'private data' section
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+is only 192 bytes per the Infiniband specification, so it's not
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+very useful for data anyway. This structure needs to remain small.
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+
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+This private data area is a convenient place to check for protocol
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+versioning because the user does not need to register memory to
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+transmit a few bytes of version information.
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+
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+This is also a convenient place to negotiate capabilities
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+(like dynamic page registration).
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+
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+If the version is invalid, we throw an error.
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+
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+If the version is new, we only negotiate the capabilities that the
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+requested version is able to perform and ignore the rest.
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+
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+Currently there is only *one* capability in Version #1: dynamic page registration
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+
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+Finally: Negotiation happens with the Flags field: If the primary-VM
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+sets a flag, but the destination does not support this capability, it
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+will return a zero-bit for that flag and the primary-VM will understand
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+that as not being an available capability and will thus disable that
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+capability on the primary-VM side.
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+
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+QEMUFileRDMA Interface:
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+=======================
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+
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+QEMUFileRDMA introduces a couple of new functions:
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+
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+1. qemu_rdma_get_buffer()  (QEMUFileOps rdma_read_ops)
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+2. qemu_rdma_put_buffer()  (QEMUFileOps rdma_write_ops)
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+
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+These two functions are very short and simply use the protocol
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+describe above to deliver bytes without changing the upper-level
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+users of QEMUFile that depend on a bytestream abstraction.
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+
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+Finally, how do we handoff the actual bytes to get_buffer()?
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+
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+Again, because we're trying to "fake" a bytestream abstraction
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+using an analogy not unlike individual UDP frames, we have
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+to hold on to the bytes received from control-channel's SEND
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+messages in memory.
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+
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+Each time we receive a complete "QEMU File" control-channel
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+message, the bytes from SEND are copied into a small local holding area.
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+
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+Then, we return the number of bytes requested by get_buffer()
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+and leave the remaining bytes in the holding area until get_buffer()
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+comes around for another pass.
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+
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+If the buffer is empty, then we follow the same steps
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+listed above and issue another "QEMU File" protocol command,
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+asking for a new SEND message to re-fill the buffer.
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+
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+Migration of pc.ram:
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+====================
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+
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+At the beginning of the migration, (migration-rdma.c),
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+the sender and the receiver populate the list of RAMBlocks
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+to be registered with each other into a structure.
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+Then, using the aforementioned protocol, they exchange a
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+description of these blocks with each other, to be used later
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+during the iteration of main memory. This description includes
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+a list of all the RAMBlocks, their offsets and lengths, virtual
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+addresses and possibly includes pre-registered RDMA keys in case dynamic
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+page registration was disabled on the server-side, otherwise not.
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+
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+Main memory is not migrated with the aforementioned protocol,
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+but is instead migrated with normal RDMA Write operations.
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+
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+Pages are migrated in "chunks" (hard-coded to 1 Megabyte right now).
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+Chunk size is not dynamic, but it could be in a future implementation.
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+There's nothing to indicate that this is useful right now.
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+
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+When a chunk is full (or a flush() occurs), the memory backed by
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+the chunk is registered with librdmacm is pinned in memory on
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+both sides using the aforementioned protocol.
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+After pinning, an RDMA Write is generated and transmitted
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+for the entire chunk.
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+
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+Chunks are also transmitted in batches: This means that we
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+do not request that the hardware signal the completion queue
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+for the completion of *every* chunk. The current batch size
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+is about 64 chunks (corresponding to 64 MB of memory).
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+Only the last chunk in a batch must be signaled.
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+This helps keep everything as asynchronous as possible
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+and helps keep the hardware busy performing RDMA operations.
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+
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+Error-handling:
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+===============
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+
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+Infiniband has what is called a "Reliable, Connected"
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+link (one of 4 choices). This is the mode in which
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+we use for RDMA migration.
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+
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+If a *single* message fails,
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+the decision is to abort the migration entirely and
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+cleanup all the RDMA descriptors and unregister all
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+the memory.
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+
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+After cleanup, the Virtual Machine is returned to normal
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+operation the same way that would happen if the TCP
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+socket is broken during a non-RDMA based migration.
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+
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+TODO:
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+=====
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+1. 'migrate x-rdma:host:port' and '-incoming x-rdma' options will be
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+   renamed to 'rdma' after the experimental phase of this work has
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+   completed upstream.
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+2. Currently, 'ulimit -l' mlock() limits as well as cgroups swap limits
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+   are not compatible with infinband memory pinning and will result in
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+   an aborted migration (but with the source VM left unaffected).
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+3. Use of the recent /proc/<pid>/pagemap would likely speed up
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+   the use of KSM and ballooning while using RDMA.
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+4. Also, some form of balloon-device usage tracking would also
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+   help alleviate some issues.
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-- 
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1.7.11.7
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