Re: [RFC,PATCH] VMWARE faults on accessing disabled counters

[Peter Zijlstra]

On Wed, Aug 31, 2016 at 02:03:58PM +0200, Jiri Olsa wrote:
> hi,
> when booting under VMWARE we've got following dmesg lines:
> 
> [    0.051567] perf_event_intel: CPUID marked event: 'cpu cycles' unavailable
> [    0.051567] perf_event_intel: CPUID marked event: 'instructions' unavailable
> [    0.051568] perf_event_intel: CPUID marked event: 'bus cycles' unavailable
> [    0.051568] perf_event_intel: CPUID marked event: 'cache references' unavailable
> [    0.051569] perf_event_intel: CPUID marked event: 'cache misses' unavailable
> [    0.051570] perf_event_intel: CPUID marked event: 'branch instructions' unavailable
> [    0.051570] perf_event_intel: CPUID marked event: 'branch misses' unavailable
> 
> that means all the architectural events are disabled by CPUID(0xa)
> 
> The kernel code sets intel_perfmon_event_map to prevent
> those event to be configured by PERF_TYPE_HARDWARE pmu
> type. However they can still be configured by via
> PERF_TYPE_RAW type.
> 
> We're getting GP fault on VMWARE when reading cycles PMC
> configured throgh the PERF_TYPE_RAW interface:
> 
>  #4 [ffff88007c603e10] do_general_protection at ffffffff8163da9e
>  #5 [ffff88007c603e40] general_protection at ffffffff8163d3a8
>     [exception RIP: native_read_pmc+6]
>     RIP: ffffffff81058d66  RSP: ffff88007c603ef0  RFLAGS: 00010083
>     RAX: ffffffff81957ee0  RBX: 0000000000000000  RCX: 0000000040000002
>     RDX: 000000000ff8f719  RSI: ffff88007c617fa8  RDI: 0000000040000002
>     RBP: ffff88007c603ef0   R8: 00007ffde5053150   R9: 0000000000000000
>     R10: 00007ffde5052530  R11: 00007fbb22aedc70  R12: ffffffff80000001
>     R13: ffff880079b74400  R14: ffff880079b74578  R15: 0000000000000010
>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0000
>  #6 [ffff88007c603ef8] x86_perf_event_update at ffffffff81029e03
>  #7 [ffff88007c603f30] x86_pmu_read at ffffffff8102a079
>  #8 [ffff88007c603f40] __perf_event_read at ffffffff811590de
> 
> I couldn't find what real HW rdpmc does on this situation,
> so I'm not sure if we actually want to prevent this.. patch
> below tries to catch this case.

Typically real hardware allows you to program any old crap. The results,
as in what the counter does, is undefined. Some actually count, some do
not.

I'm not exactly thrilled by this patch, it adds a lot of code for a
weird case. What happens when you stuff another non existing even in? GP
again?

/me mutters vile things about virt