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

["Liang, Kan" <kan.liang@linux.intel.com>]

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.


Thanks,
Kan

> +				continue;
> +
>   			wrmsrl(MSR_ARCH_PERFMON_FIXED_CTR0 + (i - INTEL_PMC_IDX_FIXED), 0);
> -		else
> +		} else {
>   			wrmsrl(x86_pmu_event_addr(i), 0);
> +		}
>   	}
>   
>   	bitmap_zero(cpuc->dirty, X86_PMC_IDX_MAX);
>