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

Jim Mattson
2023-05-24T21:33:05Z

On Wed, May 24, 2023 at 2:29 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Wed, May 24, 2023, Jim Mattson wrote:
> > 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.
>
> Per our offlist chat, my understanding is that there are caveats with vPMUs that
> it's simply not feasible for a hypervisor to handle. I.e. virtualizing any x86
> PMU with 100% accuracy isn't happening anytime soon.
>
> The way forward is likely to evaluate each caveat on a case-by-case basis to
> determine whether or not the cost of the fixup in KVM is worth the benefit to
> the guest. E.g. emulating "CPUID instructions retired" seems like it would be
> fairly straightforward. AFAICT, fixing up the VMRUN stuff is quite difficult though.

Yeah. The problem with fixing up "CPUID instructions retired" is
tracking what the event encoding is for every F/M/S out there. It's
not worth it.