Re: [PATCH] KVM: x86/pmu: Do not accidentally create BTS events

Fernand Sieber
2025-12-02T10:00:13Z

On Mon, Dec 02, 2025 at 10:35:34AM +0100, Peter Zijlstra wrote:
> On Mon, Dec 01, 2025 at 02:45:01PM +0000, Woodhouse, David wrote:
> > On Mon, 2025-12-01 at 16:23 +0200, Fernand Sieber wrote
> > > Perf considers the combination of PERF_COUNT_HW_BRANCH_INSTRUCTIONS with
> > > a sample_period of 1 a special case and handles this as a BTS event (see
> > > intel_pmu_has_bts_period()) -- a deviation from the usual semantic,
> > > where the sample_period represents the amount of branch instructions to
> > > encounter before the overflow handler is invoked.
> >
> > That's kind of awful, and seems to be the real underlying cause of the KVM
> > issue. Can we kill it with fire? Peter?
>
> Well, IIRC it gives the same information and was actually less expensive
> to run, seeing how BTS can buffer the data rather than having to take an
> interrupt on every event.
>
> But its been ages since this was done.
>
> Now arguably it should not be done for this kvm stuff, because the
> data-store buffers don't virtualize (just like old PEBS).

This. The current logic bypasses what the guest should actually be allowed
to do. See `vmx_get_supported_debugctl`, specifically the guest should not
be allowed to enable BTS.

Also semi related to this thread, but the auto enablement of BTS for
sample_period = 1 seems to yield undesirable behavior on the guest OS. The
guest OS will try to enable BTS and guest a wrmsr failure because the host
KVM rejects it, which leads to incorrect behavior (no tracing at all
happening).

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