Re: II.1 - Fixed counters on Intel

Ingo Molnar
2009-06-22T11:57:31Z

> II/ X86 comments
>
> 1/ Fixed counters on Intel
>
> You cannot simply fall back to generic counters if you cannot find
> a fixed counter. There are model-specific bugs, for instance
> UNHALTED_REFERENCE_CYCLES (0x013c), does not measure the same
> thing on Nehalem when it is used in fixed counter 2 or a generic
> counter. The same is true on Core.

This could be handled via a model specific quirk, if the erratum is
serious enough.

> You cannot simply look at the event field code to determine
> whether this is an event supported by a fixed counters. You must
> look at the other fields such as edge, invert, cnt-mask. If those
> are present then you have to fall back to using a generic counter
> as fixed counters only support priv level filtering. As indicated
> above, though, programming UNHALTED_REFERENCE_CYCLES on a generic
> counter does not count the same thing, therefore you need to fail
> if filters other than priv levels are present on this event.

Agreed, we'll fix this.