Re: [PATCH v2 4/4] perf/x86: KVM: Have perf define a dedicated struct for getting guest PEBS data

[Sean Christopherson <seanjc@google.com>]

On Fri, May 01, 2026, Jim Mattson wrote:
> On Thu, Apr 23, 2026 at 8:03 AM Sean Christopherson <seanjc@google.com> wrote:
> > diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
> > index 7403ca721b6a..04d9c51335d7 100644
> > --- a/arch/x86/events/intel/core.c
> > +++ b/arch/x86/events/intel/core.c
> > @@ -14,7 +14,6 @@
> >  #include <linux/slab.h>
> >  #include <linux/export.h>
> >  #include <linux/nmi.h>
> > -#include <linux/kvm_host.h>
> >
> >  #include <asm/cpufeature.h>
> >  #include <asm/debugreg.h>
> > @@ -4992,11 +4991,11 @@ static int intel_pmu_hw_config(struct perf_event *event)
> >   * when it uses {RD,WR}MSR, which should be handled by the KVM context,
> >   * specifically in the intel_pmu_{get,set}_msr().
> >   */
> > -static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
> > +static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr,
> > +                                                         struct x86_guest_pebs *guest_pebs)
> >  {
> >         struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> >         struct perf_guest_switch_msr *arr = cpuc->guest_switch_msrs;
> > -       struct kvm_pmu *kvm_pmu = (struct kvm_pmu *)data;
> >         u64 intel_ctrl = hybrid(cpuc->pmu, intel_ctrl);
> >         u64 pebs_mask = cpuc->pebs_enabled & x86_pmu.pebs_capable;
> >         u64 guest_pebs_mask = pebs_mask & ~cpuc->intel_ctrl_host_mask;
> > @@ -5052,7 +5051,7 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
> >          * wrong counter(s).  Similarly, disallow PEBS in the guest if the host
> >          * is using PEBS, to avoid bleeding host state into PEBS records.
> >          */
> > -       guest_pebs_mask &= kvm_pmu->pebs_enable & ~kvm_pmu->host_cross_mapped_mask;
> > +       guest_pebs_mask &= guest_pebs->enable & ~guest_pebs->cross_mapped_mask;
> 
> It would be helpful to save this mask somewhere, so that it can be
> used when calculating guest_pebs_idxs in x86_pmu_handle_guest_pebs().
> I think that code needs a fix similar to the one in commit
> 58f6217e5d01 ("perf/x86/intel: KVM: Mask PEBS_ENABLE loaded for guest
> with vCPU's value.").

Blech.  This all feels like a losing game of whack-a-mole.  Proxying the PMU
through perf is a mediocre approximation for non-PEBS events, and it seems like
it's downright awful for PEBS.  Ideally, we'd just rip out all of the perf-based
PEBS virtualization support, and only support PEBS through the mediated PMU.  :-/

Absent drastic measures though, saving the effective guest_pebs_enable in the
per-CPU tracking does seem like the least awful approach.  Though I don't quite
understand why we can't use GLOBAL_STATUS for x86_pmu_handle_guest_pebs().  I.e.
what happens if x86_pmu_handle_guest_pebs() only processes counters that actually
got marked as overflowing?

Regardless, I'm not going to try and address that mess in this series.  AFAICT,
it's not urgent, and I don't want to snowball into a broader cleanup.