[vfio-users] Huge performance decrease in VM

[Quentin Deldycke]

For adding a bit on this subjects, some games (blizzard games, tomb raider
at it's launch) use a lot of msrs during run.
It seems this is made for their anti-cheat / hack systems.

Heroes of the storm, SC2 are spamming these msrs. This reduce performance
quite hardly.

Funny thing: This "bug" does not apply to win8 / win7 :)

For reference we spoke about such in this thread a long time ago:
https://www.redhat.com/archives/vfio-users/2016-May/msg00134.html


The kernel spam of such error at Heroes of the storm login screen:
[136995.284205] kvm [6799]: vcpu2, guest rIP: 0xfffff8016e757733
kvm_set_msr_common: MSR_IA32_DEBUGCTLMSR 0x1, nop
[136995.284217] kvm [6799]: vcpu2, guest rIP: 0xfffff8016e757733
kvm_set_msr_common: MSR_IA32_DEBUGCTLMSR 0x1, nop
[137000.285330] kvm_get_msr_common: 302015 callbacks suppressed
[137000.285337] kvm [6799]: vcpu5, guest rIP: 0xfffff8016e75779c ignored
rdmsr: 0x1c9
[137000.285342] kvm [6799]: vcpu5, guest rIP: 0xfffff8016e7577aa ignored
rdmsr: 0x680
[137000.285344] kvm [6799]: vcpu5, guest rIP: 0xfffff8016e7577c1 ignored
rdmsr: 0x6c0


--
Deldycke Quentin


On 6 January 2017 at 14:07, Thomas Lindroth <thomas.lindroth at gmail.com>
wrote:

> On 01/06/2017 01:00 PM, Marius Steffen wrote:
> > Is there anything I can do to a) find the cause of this bad performance
> > b) make my VM perform better?
>
> One thing you can try is to run the game with poor performance and at the
> same time run "perf kvm --host stat live" on the host. This will show how
> many VM-EXIT kvm performs. VM-EXIT means the guest has done something the
> hardware virtualisation can't handle, like accessing MSR or virtual IO.
>
> Some workloads will perform badly because they cause a lot of VM-EXIT.
> Perf will tell you why a VM-EXIT was performed. The more common causes are
> IO_INSTRUCTION and EPT_MISCONFIG. IO_INSTRUCTION means the guest tried to
> access an x86 ioport and EPT_MISCONFIG seems to be an odd name for access
> to memory mapped io. Sometimes a lot of time is spent in HLT but that's
> normal. It only means the guest OS has nothing to do and goes idle.
>
> By running "perf kvm --host stat live --event=ioport" you can check which
> ioport is accessed and "perf kvm --host stat live --event=mmio" shows
> which address was accessed for memory mapped io.
>
> If you see that an mmio address is accessed a lot and want to find out what
> it is you can run "virsh qemu-monitor-command <nameofvm> --hmp 'info
> mtree'"
> to get a list of the memory layout of the VM. Unfortunately I don't know
> of any way to get a similar list of ioports.
>
> As an example on my system some games like Rise of the Tomb Raider and
> Assassin's Creed Unity will access ioport 0xb008 more than 200,000
> times/sec
> and those games will have poor performance compared to native. Qemu
> hardcode the acpi timer to ioport 0xb008 and those games excessively read
> the hardware timer. I haven't figured out any way to work around the
> problem
> but I run games with vsync on and even with the decreased performance I get
> 60fps in those titles.
>
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