hwpmc: Avoid using AMD PMCs if in use by firmware#2277
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Some firmwares use the PMCs to monitor OS performance. We can't be certain that the BIOS would detect any change to the counters if we reprogram them. In cases where the firmware is using the PMCs to control power management this could have dangerous side effects or unexpected performance effects. We detect if any of the counters are enabled and disable the AMD PMCs. Reported by: Sandipan Das Sponsored by: Netflix
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I need to write a similar patch for Intel. I couldn't find an AMD machine with this problem, but I have an old Intel HP box that exhibits this behavior. Sandipan is one of AMD's Linux committers that mentioned the problems in our last meeting. He cited this patch: |
mhorne
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Bad BIOS!
Generally, LGTM. You might split this logic into a helper function.
| * performance monitoring or power management functions in the BIOS to | ||
| * safely make use of the counters. | ||
| */ | ||
| for (i = 0; i < AMD_PMC_CORE_DEFAULT; i++) { |
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Should these tests come after we have queried and set amd_core_npmcs? Or is it only important to check the first 6 counters?
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It's fine were it is, we should only see this on older machines. HWP and other mechanisms are providing alternative ways for them to continue to implement similar functionality without using the counters anymore. I can't repro it on any recent Intel or AMD machines I bought from those brands that I know have done this in the past.
Don't forget it needs to come above the VM check as that will touch the first PMC.
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As for the helper function, I'll make that change and update this diff tomorrow or Saturday.
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I also owe you a patch to clean up the SYSTEM/USER flag it's going to work as you suggested, but I want to implement a best effort filtering. IBS isn't like other counters we get accurate samples but still suffer from skid during delivery.
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Some firmwares use the PMCs to monitor OS performance. We can't be certain that the BIOS would detect any change to the counters if we reprogram them. In cases where the firmware is using the PMCs to control power management this could have dangerous side effects or unexpected performance effects. We detect if any of the counters are enabled and disable the AMD PMCs.
Reported by: Sandipan Das
Sponsored by: Netflix