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[Qemu-devel] [Bug 1788665] Re: Low 2D graphics performance with Windows

From: Heiko Sieger
Subject: [Qemu-devel] [Bug 1788665] Re: Low 2D graphics performance with Windows 10 (1803) VGA passthrough VM using "Spectre" protection
Date: Thu, 30 Aug 2018 15:21:26 -0000

>The possibilities left are that either your Windows guest is lacking
software updates that could perhaps improve its performance, or that 2D
graphics really is that awful in combination with spectre/meltdown

Thanks Daniel. There are two problems with this explanation:

1. A native "bare metal" Windows 10 (1803) installation with all updates
applied does NOT have any 2D performance problems. See my attachment
(benchmarks) in the original post.

2. Both the Windows VM and the Linux VM do not see the microcode (version?), 
and therefore do not know about the Spectre vulnerability mitigation in the 
host kernel. However, the Microsoft document 
 suggests that Microsoft Hyper-V can be configured to expose new processor 
capabilities to guest virtual machines, specifically the ones made available 
through the microcode updates. Here a quote from the above Microsoft website:
"Firmware updates from your OEM may contain new processor capabilities that can 
be used to protect against CVE-2017-5715 (IBRS, STIBP, IBPB). Once the 
virtualization host's firmware has been updated, the hypervisor can make these 
additional capabilities available to guest virtual machines after taking the 
following steps."

The ideal behavior of qemu/kvm would be to expose the microcode
capabilities to the guest (I suppose this happens partially as seen by
the presence of the "pti  ssbd ibrs ibpb" flags), but obviously
something is missing.

But the real important question is this:
In the above scenario, are the VMs protected from Spectre vulnerabilities, 
simply by having the microcode installed in the host? Even when I disable the 
Spectre protection inside the Windows VM?

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  Low 2D graphics performance with Windows 10 (1803) VGA passthrough VM
  using "Spectre" protection

Status in QEMU:

Bug description:
  Windows 10 (1803) VM using VGA passthrough via qemu script.

  After upgrading Windows 10 Pro VM to version 1803, or possibly after
  applying the March/April security updates from Microsoft, the VM would
  show low 2D graphics performance (sluggishness in 2D applications and
  low Passmark results).

  Turning off Spectre vulnerability protection in Windows remedies the

  Expected behavior:
  qemu/kvm hypervisor to expose firmware capabilities of host to guest OS - see 


  Starting in March or April Microsoft began to push driver updates in
  their updates / security updates. See https://support.microsoft.com

  One update concerns the Intel microcode - see
  https://support.microsoft.com/en-us/help/4100347. It is activated by
  default within Windows.

  Once the updates are applied within the Windows guest, 2D graphics
  performance drops significantly. Other performance benchmarks are not

  A bare metal Windows installation does not display a performance loss
  after the update. See https://heiko-sieger.info/low-2d-graphics-

  Similar reports can be found here:


  6 core Intel Core i7-3930K (-MT-MCP-)

  Host OS:
  Linux Mint 19/Ubuntu 18.04
  Kernel: 4.15.0-32-generic x86_64
  Qemu: QEMU emulator version 2.11.1
  Intel microcode (host): 0x714
  dmesg | grep microcode
  [    0.000000] microcode: microcode updated early to revision 0x714, date = 
  [    2.810683] microcode: sig=0x206d7, pf=0x4, revision=0x714
  [    2.813340] microcode: Microcode Update Driver: v2.2.

  Note: I manually updated the Intel microcode on the host from 0x713 to
  0x714. However, both microcode versions produce the same result in the
  Windows guest.

  Guest OS:
  Windows 10 Pro 64 bit, release 1803

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