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

Jim Mattson
2023-05-24T20:48:09Z

On Wed, May 24, 2023 at 1:41 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Wed, Apr 26, 2023, Sandipan Das wrote:
> > Hi Sean, Like,
> >
> > On 4/19/2023 7:11 PM, Like Xu wrote:
> > >> 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?ï¿½ 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.
> > >
> >
> > This behaviour is architectural and an erratum will not be issued. However, for clarity, a future
> > release of the APM will include additional details like the following:
> >
> > 1) From the perspective of performance monitoring counters, VMRUNs are considered as far control
> > transfers and VMEXITs as exceptions.
> >
> > 2) When the performance monitoring counters are set up to count events only in certain modes
> > through the "OsUserMode" and "HostGuestOnly" bits, instructions and events that change the
> > mode are counted in the target mode. For example, a SYSCALL from CPL 3 to CPL 0 with a
> > counter set to count retired instructions with USR=1 and OS=0 will not cause an increment of
> > the counter. However, the SYSRET back from CPL 0 to CPL 3 will cause an increment of the
> > counter and the total count will end up correct. Similarly, when counting PMCx0C6 (retired
> > far control transfers, including exceptions and interrupts) with Guest=1 and Host=0, a VMRUN
> > instruction will cause an increment of the counter. However, the subsequent VMEXIT that occurs,
> > since the target is in the host, will not cause an increment of the counter and so the total
> > count will end up correct.
>
> The count from the guest's perspective does not "end up correct". Unlike SYSCALL,
> where _userspace_ deliberately and synchronously executes a branch instruction,
> VMEXIT and VMRUN are supposed to be transparent to the guest and can be completely
> asynchronous with respect to guest code execution, e.g. if the host is spamming
> IRQs, the guest will see a potentially large number of bogus (from it's perspective)
> branches retired.

The reverse problem occurs when a PMC is configured to count "CPUID
instructions retired." Since KVM intercepts CPUID and emulates it, the
PMC will always read 0, even if the guest executes a tight loop of
CPUID instructions.

The PMU is not virtualizable on AMD CPUs without significant
hypervisor corrections. I have to wonder if it's really worth the
effort.