[Qemu-devel] [Bug 1788665] Re: Low 2D graphics performance with Windows 10 (1803) VGA passthrough VM using "Spectre" protection

[Heiko Sieger]

Whew, after some hurdles I managed to install a Linux Mint 19 guest
(Ubuntu 18.04). After all updates, here the output:

$ dmesg | grep microcode
[    0.036780] core: PEBS disabled due to CPU errata, please upgrade microcode

So the microcode in the guest is not loaded! But see below:

$ cat /proc/cpuinfo | grep flags
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov 
pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx pdpe1gb rdtscp lm 
constant_tsc arch_perfmon rep_good nopl xtopology cpuid pni pclmulqdq vmx ssse3 
cx16 pcid sse4_1 sse4_2 x2apic popcnt tsc_deadline_timer aes xsave avx 
hypervisor lahf_lm cpuid_fault pti ssbd ibrs ibpb tpr_shadow vnmi flexpriority 
ept vpid tsc_adjust xsaveopt arat

Here is the qemu command I use for this Linux guest (it is almost
identical to the Windows 10 VM command):

qemu-system-x86_64 \
  -runas user \
  -monitor stdio \
  -serial none \
  -parallel none \
  -nodefaults \
  -nodefconfig \
  -name $vmname,process=$vmname \
  -machine q35,accel=kvm,kernel_irqchip=on \
-cpu 
host,kvm=off,hv_vendor_id=1234567890ab,hv_vapic,hv_time,hv_relaxed,hv_spinlocks=0x1fff
 \
  -smp 6,sockets=1,cores=3,threads=2 \
  -m 16G \
  -mem-path /dev/hugepages \
  -mem-prealloc \
  -balloon none \
  -rtc base=localtime,clock=host \
  -vga none \
  -nographic \
  -soundhw hda \
  -device vfio-pci,host=02:00.0,multifunction=on \
  -device vfio-pci,host=02:00.1 \
  -device vfio-pci,host=00:1a.0 \
  -device vfio-pci,host=08:00.0 \
  -drive if=pflash,format=raw,readonly,file=/usr/share/OVMF/OVMF_CODE.fd \
  -drive if=pflash,format=raw,file=/tmp/my_vars.fd \
  -boot order=c \
  -drive id=disk0,if=virtio,cache=none,format=raw,file=/home/user/win.img \
  -netdev type=tap,id=net0,ifname=vmtap0,vhost=on \
  -device virtio-net-pci,netdev=net0,mac=00:16:3e:00:01:01


Kernel: 4.15.0-33-generic x86_64

$ grep microcode /proc/cpuinfo
microcode       : 0x1
microcode       : 0x1
microcode       : 0x1
microcode       : 0x1
microcode       : 0x1
microcode       : 0x1

In essence:
The microcode is NOT loaded in the Linux VM. However, the following feature 
flags are listed: "pti ssbd ibrs ibpb". The "stibp" flag is missing.

Hope this helps.

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https://bugs.launchpad.net/bugs/1788665

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

Status in QEMU:
  New

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
  issue.

  Expected behavior:
  qemu/kvm hypervisor to expose firmware capabilities of host to guest OS - see 
https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/CVE-2017-5715-and-hyper-v-vms

  Background:

  Starting in March or April Microsoft began to push driver updates in
  their updates / security updates. See https://support.microsoft.com
  /en-us/help/4073757/protect-your-windows-devices-against-spectre-
  meltdown

  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
  affected.

  A bare metal Windows installation does not display a performance loss
  after the update. See https://heiko-sieger.info/low-2d-graphics-
  benchmark-with-windows-10-1803-kvm-vm/

  Similar reports can be found here:
  
https://www.reddit.com/r/VFIO/comments/97unx4/passmark_lousy_2d_graphics_performance_on_windows/

  Hardware:

  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 = 
2018-05-08
  [    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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