linux-kernel - Re: [perf] fuzzer triggers unchecked MSR access error: WRMSR to 0x318

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Message-ID: <YQLJhAiYQKtvvhjJ@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Jul 2021 17:30:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Vince Weaver <vincent.weaver@...ne.edu>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        "Liang, Kan" <kan.liang@...el.com>
Subject: Re: [perf] fuzzer triggers unchecked MSR access error: WRMSR to 0x318

On Thu, Jul 29, 2021 at 09:21:01AM -0400, Liang, Kan wrote:
> 
> 
> On 7/29/2021 5:14 AM, Peter Zijlstra wrote:
> > On Wed, Jul 28, 2021 at 12:49:43PM -0400, Vince Weaver wrote:
> > > [32694.087403] unchecked MSR access error: WRMSR to 0x318 (tried to write 0x0000000000000000) at rIP: 0xffffffff8106f854 (native_write_msr+0x4/0x20)
> > > [32694.101374] Call Trace:
> > > [32694.103974]  perf_clear_dirty_counters+0x86/0x100
> > 
> > Hmm.. if I read this right that's MSR_ARCH_PERFMON_FIXED_CTR0 + i, given
> > that FIXED_CTR0 is 0x309 that gives i == 15, which is FIXED_BTS.
> > 
> > I'm thinking something like this ought to cure things.
> > 
> > ---
> >   arch/x86/events/core.c | 12 +++++++-----
> >   1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > index 1eb45139fcc6..04edf8017961 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -2489,13 +2489,15 @@ void perf_clear_dirty_counters(void)
> >   		return;
> >   	for_each_set_bit(i, cpuc->dirty, X86_PMC_IDX_MAX) {
> > -		/* Metrics and fake events don't have corresponding HW counters. */
> > -		if (is_metric_idx(i) || (i == INTEL_PMC_IDX_FIXED_VLBR))
> > -			continue;
> > -		else if (i >= INTEL_PMC_IDX_FIXED)
> > +		if (i >= INTEL_PMC_IDX_FIXED) {
> > +			/* Metrics and fake events don't have corresponding HW counters. */
> > +			if ((i - INTEL_PMC_IDX_FIXED) >= x86_pmu.num_counters_fixed)
> 
> Yes, the fix is better. My previous implementation tries to pick up all the
> special cases. It's very likely to miss some special cases like FIXED_BTS
> and probably any new fake events added later if there are.
> Thanks for the fix.
> 
> The x86_pmu.num_counters_fixed should work well on HSW. But we have hybrid
> machines now. I think we can use
> hybrid(cpuc->pmu, num_counters_fixed) instead, which should be more
> accurate.

Yes, good point. I guess I still need to adjust to the hybrid world.