On Thu, Aug 29, 2019 at 12:56:02PM -0400, Liang, Kan wrote: > On 8/29/2019 9:52 AM, Peter Zijlstra wrote:
> > What what? The PERF_METRICS contents depends on the FIXCTR3 value ?! > > Yes. > > For current implementation, PERF_METRIC MSR is composed by four fields, > backend bound, frontend bound, bad speculation and retiring. > Each of the fields are populated using the below formula for eg: > PERF_METRIC[RETIRING] = (0xFF * > PERF_METRICS_RETIRING_INTERNAL_48bit_COUNTER) > / FIXCTR3 So it really depends on the actual exposed FIXCTR3 _value_ to compute the PERF_METRIC field? *mind boggles*, that's really unspeakable crap. And this isn't documented anywhere afaict. I was thinking they've have an internal counter for the SLOTS value too, so the PERF_METRIC fields are indenpendent; which would be like 'sane'. Exposing the internal counters would've been _soooo_ much better, just add 4 more fixed counters and call it a day. > The METRICS_OVF indicates the overflow of any internal counters. OK, but I'm thinking that by that time the fraction in PERF_METRIC will be too coarse and we're loosing precision. Reconstruction will be inaccurate. > The internal counters only start counting from 0, which cannot be programmed > by SW. But resetting the PERF_METRIC would implicitly resetting the internal > counters. The only possible option given the choices.