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

From: Like Xu
Date: Mon Aug 02 2021 - 07:41:08 EST

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)
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

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>