Re: [PATCH] perf/x86/kvm: Fix Broadwell Xeon stepping in isolation_ucodes[]

[Andi Kleen]

On Wed, Apr 21, 2021 at 05:18:34PM -0700, Jim Mattson wrote:
> The only stepping of Broadwell Xeon parts is stepping 1. Fix the
> relevant isolation_ucodes[] entry, which previously enumerated
> stepping 2.
> 
> Although the original commit was characterized as an optimization, it
> is also a workaround for a correctness issue.
> 
> If a PMI arrives between kvm's call to perf_guest_get_msrs() and the
> subsequent VM-entry, a stale value for the IA32_PEBS_ENABLE MSR may be
> restored at the next VM-exit. This is because, unbeknownst to kvm, PMI
> throttling may clear bits in the IA32_PEBS_ENABLE MSR. CPUs with "PEBS
> isolation" don't suffer from this issue, because perf_guest_get_msrs()
> doesn't report the IA32_PEBS_ENABLE value.

Acked-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>

-Andi