Re: [RFC PATCH 1/2] KVM: x86/pmu: Add CAP to disable SW accounting of emulated instructions

[Sean Christopherson]

On Mon, Jul 20, 2026, Luka Absandze wrote:
> The only functional change for an opted-in VM is reduced accuracy: a
> guest counting instructions-retired or branches-retired undercounts by
> the instructions KVM emulates in host context, i.e. the behavior that
> predates the accounting cited above. Hardware-executed guest
> instructions continue to be counted by the backing perf_event, and its
> overflow/PMI path is unchanged.

Do you *need* per-VM control, or would a module param (or a magic value for
enable_pmu) suffice?  While I mostly buy the "it used to work this way" argument
(just "mostly", because that commit landed 4.5 years ago), I'm not exactly keen
on adding uAPI that is effectively "re-introduce a bug to workaround fundamental
design issues in KVM's emulated PMU implementation".

If we do add proper uAPI, we should extend KVM_CAP_PMU_CAPABILITY, not add a new
CAP entirely.