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

[Like Xu]

On 29/7/2021 11:30 pm, Peter Zijlstra wrote:
> 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.

I recently enabled guest BTS and encountered the same call trace.

For the proposal hybrid(cpuc->pmu, num_counters_fixed),
Tested-by: Like Xu <likexu@xxxxxxxxxxx>