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From 332f7a6f8dd4ee6fae95c87e717d4527ca9344a7 Mon Sep 17 00:00:00 2001
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From: Alex Williamson <alex.williamson@redhat.com>
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Date: Tue, 13 Feb 2018 19:04:56 +0100
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Subject: [PATCH 15/15] Disable GeForce quirks in vfio-pci for RHEL machines
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RH-Author: Alex Williamson <alex.williamson@redhat.com>
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Message-id: <20180213190456.27565.18217.stgit@gimli.home>
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Patchwork-id: 78999
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O-Subject: [RHEL-7.5 qemu-kvm-rhev PATCH v2 2/2] [RHEL] Disable GeForce quirks in vfio-pci for RHEL machines
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Bugzilla: 1508330
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RH-Acked-by: Laszlo Ersek <lersek@redhat.com>
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RH-Acked-by: Auger Eric <eric.auger@redhat.com>
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RH-Acked-by: Marcel Apfelbaum <marcel@redhat.com>
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RHEL-ONLY - The GeForce quirks are primarily used to virtualize the
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various ways the driver and VGA BIOS has available to access PCI
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config space using alternate memory regions, ex. MMIO and I/O port.
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Generally these quirks are benign and even where not required, do not
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interfere with operation. However, NVIDIA has a strange MSI behavior
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where the MSI interrupt will not re-trigger until the driver writes
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back to specifc addresses in config space via the MMIO mirror. Since
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we virtualize the mirror for GeForce, every MSI interrupt requires an
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exit to QEMU to handle this MSI-ACK behavior. For very high interrupt
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rate applications, such as NVIDIA's DolphinVS test program, this
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virtualization causes enough overhead to degrade the performance.
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RHEL does not support GeForce assignment and supported configurations
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using Quadro, GRID, or Tesla assignment, including NVIDIA vGPU, do not
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require these quirks. We can therefore disable these quirks by
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default, removing this overhead for supported configurations while
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still allowing unsupported configurations access to re-enable them
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via (equally unsupported) <qemu:args> options.
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Upstream development is underway to mitigate this performance overhead
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even for GeForce, but preliminary results show that we can only
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achieve 95% of the performance of disabling the quirk entirely via an
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ioeventfd approach. Obviously detecting GeForce vs Quadro would also
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be a useful approach, but there's no clear way to do this aside from
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scraping device descriptions from libpci. Therefore, disabling
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GeForce quirks, with unsupported mechanisms to re-enable unsupported
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configurations, seems like the best approach for RHEL/RHV.
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Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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include/hw/i386/pc.h | 5 +++++
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1 file changed, 5 insertions(+)
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diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
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index d1b1320..2b35ed8 100644
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--- a/include/hw/i386/pc.h
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+++ b/include/hw/i386/pc.h
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@@ -1002,6 +1002,11 @@ extern void igd_passthrough_isa_bridge_create(PCIBus *bus, uint16_t gpu_dev_id);
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.driver = TYPE_X86_CPU,\
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.property = "host-phys-bits",\
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.value = "on",\
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+ },\
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+ { /* PC_RHEL_COMPAT bz 1508330 */ \
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+ .driver = "vfio-pci",\
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+ .property = "x-no-geforce-quirks",\
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+ .value = "on",\
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},
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#define PC_RHEL7_4_COMPAT \
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--
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1.8.3.1
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