[vfio-users] Huge performance decrease in VM

[Marius Steffen]

I think I've read about this on this mailing list, because when playing 
Crysis 3, my VM kept crashing (more precise: the game didn't even 
start), until I set 'options kvm ignore_msrs=1' in modprobe.d .conf file.
When switching to Win8, will the performance be better?


Thanks,

Marius


Am 06.01.2017 um 14:52 schrieb 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 
> <mailto: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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