On 05/21/2015 06:57 PM, George Dunlap wrote:
> On 05/18/2015 08:16 PM, 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.
>>
>> 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).
>>
>> Signed-off-by: Don Zickus <dzic...@redhat.com>
>
> Right -- so what was actually broken was the "does this register work"
> check, which needs a non-enabled register.
>
> Would it make sense to add a comment somewhere in the code saying that
> you need a disabled event counter for the MSR check to work properly?
> It's sort of implied but it's not explicit.
>
> Other than that, this looks good to me. I'm not positive I have access
> to the box I needed this for anymore -- I'll take a look for it next week.
>
> In the mean time:
>
> Acked-by: George Dunlap <george.dun...@eu.citrix.com>
I managed to track down the machine that had the problem and verify that
things still work for me after this patch. So now you can add:
Tested-by: George Dunlap <george.dun...@eu.citrix.com>
Thanks,
-George
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