perf/x86/intel: Don't write PEBS_ENABLED on host<=>guest xfers if CPU has PEBS isolation, to fix stuck PEBS_ENABLED

When filling the list of MSRs to be loaded by KVM on VM-Enter and VM-Exit,
*never* insert an entry for PEBS_ENABLED if the CPU properly isolates PEBS
events, in which case disabling counters via PERF_GLOBAL_CTRL is sufficient
to prevent unwanted PEBS events in the guest (or host).  Because perf loads
PEBS_ENABLE with the unfiltered cpu_hw_events.pebs_enabled, i.e. with both
host and guest masks, there is no need to load different values for the
guest versus host, perf+KVM can and should simply control which counters
are enabled/disabled via PERF_GLOBAL_CTRL.

Avoiding touching PEBS_ENABLED "fixes" a bug where PEBS_ENABLED can end up
with "stuck" bits if a PEBS event is throttled between generating the list
and actually entering the guest (Intel CPUs can't arbtitrarily block NMIs).
Fixes in quotes because leaving PEBS_ENABLED as-is doesn't fix the
underlying problem of perf (via PMIs) being able to modify state after the
perf<=>KVM handoff.

But not writing PEBS_ENABLED is desirable no matter what, as stating the
obvious, leaving PEBS_ENABLED as-is avoids three MSR writes on every VMX
transition: one each on entry/exit, and one more explicit WRMSR to zero
PEBS_ENABLED before VM-Entry (KVM assumes the only reason PEBS_ENABLED is
in the load list is if the CPU lacks PEBS isolation and thus needs a
quiescent period).

Opportunistically add comments to (better) explain the rules for generating
the set of PEBS counters that will be active while the guest is running,
along with a FIXME for the suspected hack-a-fix where perf disables guest
PEBS if _any_ PEBS event is configured to count in the host (commit
854250329c02 ("KVM: x86/pmu: Disable guest PEBS temporarily in two rare
situations") doesn't explain the motivation, at all).

Fixes: c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://patch.msgid.link/20260921191418.950933-3-seanjc@google.com

0 files changed, 0 insertions, 0 deletions