Re: [PATCH] x86, perf: Tweak broken BIOS rules during check_hw_exists

[Peter Zijlstra]

On Mon, May 18, 2015 at 03:16:48PM -0400, Don Zickus wrote:
> I stumbled upon an AMD box that had the BIOS using a hardware counter.  Instead
> of printing out a warning and continuing, it failed and blocked further perf
> counter usage.

Hehe, which was the original behaviour iirc.

> Looking through the history, I found commit a5ebe0ba3dff had tweaked the rules
> for a xen guest on an almost identical box and now changed the behaviour.
> 
> Unfortunately the rules were tweaked incorrectly and will always lead to msr
> failures even though the msrs are completely fine.
> 
> What happens now is in arch/x86/kernel/cpu/perf_event.c::check_hw_exists:
> 
> <snip>
>         for (i = 0; i < x86_pmu.num_counters; i++) {
>                 reg = x86_pmu_config_addr(i);
>                 ret = rdmsrl_safe(reg, &val);
>                 if (ret)
>                         goto msr_fail;
>                 if (val & ARCH_PERFMON_EVENTSEL_ENABLE) {
>                         bios_fail = 1;
>                         val_fail = val;
>                         reg_fail = reg;
>                 }
>         }
> 
> <snip>
>         /*
>          * Read the current value, change it and read it back to see if it
>          * matches, this is needed to detect certain hardware emulators
>          * (qemu/kvm) that don't trap on the MSR access and always return 0s.
>          */
>         reg = x86_pmu_event_addr(0);
> 				^^^^
> 
> if the first perf counter is enabled, then this routine will always fail
> because the counter is running. :-(
> 
>         if (rdmsrl_safe(reg, &val))
>                 goto msr_fail;
>         val ^= 0xffffUL;
>         ret = wrmsrl_safe(reg, val);
>         ret |= rdmsrl_safe(reg, &val_new);
>         if (ret || val != val_new)
>                 goto msr_fail;
> 
> The above bios_fail used to be a 'goto' which is why it worked in the past.
> 
> Further, most vendors have migrated to using fixed counters to hide their
> evilness hence this problem rarely shows up now days except on a few old boxes.
> 
> I fixed my problem and kept the spirit of the original Xen fix, by recording a
> safe non-enable register to be used safely for the reading/writing check.
> Because it is not enabled, this passes on bare metal boxes (like metal), but
> should continue to throw an msr_fail on Xen guests because the register isn't
> emulated yet.
> 
> Now I get a proper bios_fail error message and Xen should still see their
> msr_fail message (untested).

Thanks!
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