Re: [PATCH 5/5] KVM: x86/pmu: Hide guest counter updates from the VMRUN instruction

Like Xu
2023-04-19T13:42:01Z

On 7/4/2023 10:56 pm, Sean Christopherson wrote:
> On Fri, Apr 07, 2023, Like Xu wrote:
>> On 7/4/2023 10:18 am, Sean Christopherson wrote:
>>> Wait, really? VMRUN is counted if and only if it enters to a CPL0 guest? Can
>>> someone from AMD confirm this? I was going to say we should just treat this as
>>> "normal" behavior, but counting CPL0 but not CPL>0 is definitely quirky.
>>
>> VMRUN is only counted on a CPL0-target (branch) instruction counter.
>
> Yes or no question: if KVM does VMRUN and a PMC is programmed to count _all_ taken
> branches, will the PMC count VMRUN as a branch if guest CPL>0 according to the VMCB?

YES, my quick tests (based on run_in_user() from KUT on Zen4) show:

EVENTSEL_GUESTONLY + EVENTSEL_ALL + VMRUN_to_USR -> AMD_ZEN_BR_RETIRED + 1
EVENTSEL_GUESTONLY + EVENTSEL_ALL + VMRUN_to_OS -> AMD_ZEN_BR_RETIRED + 1

EVENTSEL_GUESTONLY + EVENTSEL_USR + VMRUN_to_USR -> AMD_ZEN_BR_RETIRED + 1
EVENTSEL_GUESTONLY + EVENTSEL_OS + VMRUN_to_OS -> AMD_ZEN_BR_RETIRED + 1

VENTSEL_GUESTONLY + EVENTSEL_OS + VMRUN_to_USR -> No change
VENTSEL_GUESTONLY + EVENTSEL_USR + VMRUN_to_OS -> No change

I'm actually not surprised and related test would be posted later.

>
>> This issue makes a guest CPL0-target instruction counter inexplicably
>> increase, as if it would have been under-counted before the virtualization
>> instructions were counted.
>
> Heh, it's very much explicable, it's just not desirable, and you and I would argue
> that it's also incorrect.

This is completely inaccurate from the end guest pmu user's perspective.

I have a toy that looks like virtio-pmu, through which guest users can get
hypervisor performance data.
But the side effect of letting the guest see the VMRUN instruction by default is
unacceptable, isn't it ?

>
> AMD folks, are there plans to document this as an erratum? I agree with Like that
> counting VMRUN as a taken branch in guest context is a CPU bug, even if the behavior
> is known/expected.

+CC: Santosh, Tom, Ananth