From a3df1677bfe1f2ae0a6bcb54f745b08b211369ce Mon Sep 17 00:00:00 2001

From: Laurent Vivier <lvivier@redhat.com>

Date: Wed, 7 Feb 2018 15:47:54 +0100

Subject: [PATCH 02/15] migration/savevm.c: set MAX_VM_CMD_PACKAGED_SIZE to 1ul

 << 32

RH-Author: Laurent Vivier <lvivier@redhat.com>

Message-id: <20180207154754.9064-1-lvivier@redhat.com>

Patchwork-id: 78921

O-Subject: [RHV7.5 qemu-kvm-rhev PATCH] migration/savevm.c: set MAX_VM_CMD_PACKAGED_SIZE to 1ul << 32

Bugzilla: 1540003

RH-Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

RH-Acked-by: David Gibson <dgibson@redhat.com>

RH-Acked-by: Thomas Huth <thuth@redhat.com>

From: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>

MAX_VM_CMD_PACKAGED_SIZE is a constant used in qemu_savevm_send_packaged

and loadvm_handle_cmd_packaged to determine whether a package is too

big to be sent or received. qemu_savevm_send_packaged is called inside

postcopy_start (migration/migration.c) to send the MigrationState

in a single blob to the destination, using the MIG_CMD_PACKAGED subcommand,

which will read it up using loadvm_handle_cmd_packaged. If the blob is

larger than MAX_VM_CMD_PACKAGED_SIZE, an error is thrown and the postcopy

migration is aborted. Both MAX_VM_CMD_PACKAGED_SIZE and MIG_CMD_PACKAGED

were introduced by commit 11cf1d984b ("MIG_CMD_PACKAGED: Send a packaged

chunk ..."). The constant has its original value of 1ul << 24 (16MB).

The current MAX_VM_CMD_PACKAGED_SIZE value is not enough to support postcopy

migration of bigger pseries guests. The blob size for a postcopy migration of

a pseries guest with the following setup:

qemu-system-ppc64 --nographic -vga none -machine pseries,accel=kvm -m 64G \

-smp 1,maxcpus=32 -device virtio-blk-pci,drive=rootdisk \

-drive file=f27.qcow2,if=none,cache=none,format=qcow2,id=rootdisk \

-netdev user,id=u1 -net nic,netdev=u1

Goes around 12MB. Bumping the RAM to 128G makes the blob sizes goes to 20MB.

With 256G the blob goes to 37MB - more than twice the current maximum size.

At this moment the pseries machine can handle guests with up to 1TB of RAM,

making this postcopy blob goes to 128MB of size approximately.

Following the discussions made in [1], there is a need to understand what

devices are aggressively consuming the blob in that manner and see if that

can be mitigated. Until then, we can set MAX_VM_CMD_PACKAGED_SIZE to the

maximum value allowed. Since the size is a 32 bit int variable, we can set

it as 1ul << 32, giving a maximum blob size of 4G that is enough to support

postcopy migration of 32TB RAM guests given the above constraints.

[1] https://lists.nongnu.org/archive/html/qemu-devel/2018-01/msg06313.html

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>

Reported-by: Balamuruhan S <bala24@linux.vnet.ibm.com>

Reviewed-by: Juan Quintela <quintela@redhat.com>

Signed-off-by: Juan Quintela <quintela@redhat.com>

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

(cherry picked from commit ee555cdf4d495ddd83633406e3099c5d1ef22e0a)

Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

---

 migration/savevm.c | 2 +-

 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/migration/savevm.c b/migration/savevm.c

index 5eb3504..c466921 100644

--- a/migration/savevm.c

+++ b/migration/savevm.c

@@ -83,7 +83,7 @@ enum qemu_vm_cmd {

 };

 bool shadow_bios_after_incoming;

-#define MAX_VM_CMD_PACKAGED_SIZE (1ul << 24)

+#define MAX_VM_CMD_PACKAGED_SIZE UINT32_MAX

 static struct mig_cmd_args {

     ssize_t     len; /* -1 = variable */

     const char *name;

-- 

1.8.3.1