Re: [PATCH v3 7/9] perf/x86/amd: Enable VPMU passthrough capability for IBS PMU

[Jim Mattson <jmattson@google.com>]

On Tue, Mar 10, 2026 at 06:00:19AM +0000, Manali Shukla wrote:
> IBS MSRs are classified as Swap Type C, which requires the hypervisor
> to save and restore its own IBS state before VMENTRY and after VMEXIT.
>
> To support this, set the ibs_op and ibs_fetch PMUs with the
> PERF_PMU_CAP_MEDIATED_VPMU capability. This ensures that these PMUs are
> exclusively owned by the guest while it is running, allowing the
> hypervisor to manage IBS state transitions correctly.
>
> Signed-off-by: Manali Shukla <manali.shukla@amd.com>
> ---
>  arch/x86/events/amd/ibs.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index 6d230e417b61..e075c5ed136c 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -978,6 +978,7 @@ static struct perf_ibs perf_ibs_fetch = {
>  		.stop		= perf_ibs_stop,
>  		.read		= perf_ibs_read,
>  		.check_period	= perf_ibs_check_period,
> +		.capabilities	= PERF_PMU_CAP_MEDIATED_VPMU,

At this point in the series, perf_ibs_init() still rejects events with
exclude_guest set, so every IBS event is an "include guest" event, as
defined by is_include_guest_event(). With this capability:

- If any IBS event exists, perf_create_mediated_pmu() returns -EBUSY, so
  KVM cannot create a vCPU in a VM with a mediated PMU.

- If any VM with a mediated PMU exists, mediated_pmu_account_event()
  returns -EOPNOTSUPP, so no IBS event can be created.

Patch 8/9 removes the exclude_guest check. Please squash patches 7/9 and
8/9 together, so that the series does not have this regression at any
commit.

Do we really want to set this capability when the CPU does not support IBS
virtualization (or when kvm_amd.vibs=0)? In that case, the guest cannot use
IBS, but host IBS events that do not set exclude_guest still block mediated
PMU VMs, and vice versa.

When the guest context is loaded, perf_load_guest_context() schedules
out the exclude_guest IBS events through perf_ibs_stop(). On hardware
with IBS_CAPS_DIS, perf_ibs_stop() does not clear IbsFetchEn or IbsOpEn.
See my comment on patch 6/9.

>  	},
>  	.msr			= MSR_AMD64_IBSFETCHCTL,
>  	.msr2			= MSR_AMD64_IBSFETCHCTL2,