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Message-ID: <ZUrCE9j1Gvw4zacY@google.com>
Date: Tue, 7 Nov 2023 15:02:43 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Mingwei Zhang <mizhang@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
Like Xu <likexu@...cent.com>, Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] perf/x86: Don't enforce minimum period for KVM guest-only events
On Tue, Nov 07, 2023, Mingwei Zhang wrote:
> On Tue, Nov 07, 2023, Sean Christopherson wrote:
> > arch/x86/events/core.c | 21 +++++++++++++++------
> > 1 file changed, 15 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > index 40ad1425ffa2..f8a8a4ea4d47 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -1388,16 +1388,25 @@ int x86_perf_event_set_period(struct perf_event *event)
> > hwc->last_period = period;
> > ret = 1;
> > }
> > - /*
> > - * Quirk: certain CPUs dont like it if just 1 hw_event is left:
> > - */
> > - if (unlikely(left < 2))
> > - left = 2;
> >
> > if (left > x86_pmu.max_period)
> > left = x86_pmu.max_period;
> >
> > - static_call_cond(x86_pmu_limit_period)(event, &left);
> > + /*
> > + * Exempt KVM guest events from the minimum period requirements. It's
> > + * the guest's responsibility to ensure it can make forward progress,
> > + * and it's KVM's responsibility to configure an appropriate "period"
> > + * to correctly virtualize overflow for the guest's PMCs.
> > + */
> > + if (!event->attr.exclude_host) {
> > + /*
> > + * Quirk: certain CPUs dont like it if just 1 event is left:
> > + */
> > + if (unlikely(left < 2))
> > + left = 2;
> > +
> > + static_call_cond(x86_pmu_limit_period)(event, &left);
> > + }
> >
> > this_cpu_write(pmc_prev_left[idx], left);
> >
>
> Nice one. I am curious how you tested this one? I would like to
> reproduce that one on my side.
The check_emulated_instr() sub-test in KVM-Unit-Tests's x86/pmu.c fails when run
with "my" (which is really yours) fix for the KVM's handling of emulated PMC
events[*]. If KVM synthesizes an "instructions retired" event that bumps the
PMC to all ones, i.e. -1 for all intents and purposes, the test fails because
KVM creates a sample_period of '1', but perf programs a period of '2'.
I suspect a very simple test of writing -1 to a PMC from the guest would exhibit
the same behavior.
[*] https://lkml.kernel.org/r/ZUWAg3WP2XESCAR4%40google.com
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