Re: [RFC PATCH 01/41] perf: x86/intel: Support PERF_PMU_CAP_VPMU_PASSTHROUGH

[Sean Christopherson]

On Thu, Apr 11, 2024, Jim Mattson wrote:
> On Thu, Apr 11, 2024 at 10:21 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
> > On 2024-04-11 1:04 p.m., Sean Christopherson wrote:
> > > On Fri, Jan 26, 2024, Xiong Zhang wrote:
> > >> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> > >>
> > >> Define and apply the PERF_PMU_CAP_VPMU_PASSTHROUGH flag for the version 4
> > >> and later PMUs
> > >
> > > Why?  I get that is an RFC, but it's not at all obvious to me why this needs to
> > > take a dependency on v4+.
> >
> > The IA32_PERF_GLOBAL_STATUS_RESET/SET MSRs are introduced in v4. They
> > are used in the save/restore of PMU state. Please see PATCH 23/41.
> > So it's limited to v4+ for now.
> 
> Prior to version 4, semi-passthrough is possible, but IA32_PERF_GLOBAL_STATUS
> has to be intercepted and emulated, since it is non-trivial to set bits in
> this MSR.

Ah, then this _perf_ capability should be PERF_PMU_CAP_WRITABLE_GLOBAL_STATUS or
so, especially since it's introduced in advance of the KVM side of things.  Then
whether or not to support a mediated PMU becomes a KVM decision, e.g. intercepting
accesses to IA32_PERF_GLOBAL_STATUS doesn't seem like a complete deal breaker
(or maybe it is, I now see the comment about it being used to do the context switch).

And peeking ahead, IIUC perf effectively _forces_ a passthrough model when
has_vpmu_passthrough_cap() is true, which is wrong.  There needs to be a user/admin
opt-in (or opt-out) to that behavior, at a kernel/perf level, not just at a KVM
level.  Hmm, or is perf relying on KVM to do that right thing?  I.e. relying on
KVM to do perf_guest_{enter,exit}() if and only if the PMU can support the
passthrough model.

If that's the case, most of the has_vpmu_passthrough_cap() checks are gratiutous
and confusing, e.g. just WARN if KVM (or some other module) tries to trigger a
PMU context switch when it's not supported by perf.