linux-kernel - Re: [PATCH] x86/perf: Fix guest_get_msrs static call if there is no PMU

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Message-ID: <YEXmILSHDNDuMk/N@hirez.programming.kicks-ass.net>
Date:   Mon, 8 Mar 2021 09:53:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Xu, Like" <like.xu@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Like Xu <like.xu@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>, kvm@...r.kernel.org,
        "Thomas Gleixner
        (x86/pti/timer/core/smp/irq/perf/efi/locking/ras/objtool)
        (x86@...nel.org)" <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] x86/perf: Fix guest_get_msrs static call if there is no
 PMU

On Mon, Mar 08, 2021 at 10:25:59AM +0800, Xu, Like wrote:
> On 2021/3/6 6:33, Sean Christopherson wrote:
> > Handle a NULL x86_pmu.guest_get_msrs at invocation instead of patching
> > in perf_guest_get_msrs_nop() during setup.  If there is no PMU, setup
> 
> "If there is no PMU" ...

Then you shouldn't be calling this either ofcourse :-)

> > @@ -671,7 +671,11 @@ void x86_pmu_disable_all(void)
> >   struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
> >   {
> > -	return static_call(x86_pmu_guest_get_msrs)(nr);
> > +	if (x86_pmu.guest_get_msrs)
> > +		return static_call(x86_pmu_guest_get_msrs)(nr);
> 
> How about using "static_call_cond" per commit "452cddbff7" ?

Given the one user in atomic_switch_perf_msrs() that should work because
it doesn't seem to care about nr_msrs when !msrs.

Still, it calling atomic_switch_perf_msrs() and
intel_pmu_lbr_is_enabled() when there isn't a PMU at all is of course, a
complete waste of cycles.