Re: [PATCH V5 RESEND 00/14] TopDown metrics support for Icelake

[Peter Zijlstra]

On Mon, Jan 06, 2020 at 12:29:05PM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> Icelake has support for measuring the level 1 TopDown metrics
> directly in hardware. This is implemented by an additional METRICS
> register, and a new Fixed Counter 3 that measures pipeline SLOTS.
> 
> New in Icelake
> - Do not require generic counters. This allows to collect TopDown always
>   in addition to other events.
> - Measuring TopDown per thread/process instead of only per core
> 
> For the Ice Lake implementation of performance metrics, the values in
> PERF_METRICS MSR are derived from fixed counter 3. Software should start
> both registers, PERF_METRICS and fixed counter 3, from zero.
> Additionally, software is recommended to periodically clear both
> registers in order to maintain accurate measurements. The latter is
> required for certain scenarios that involve sampling metrics at high
> rates. Software should always write fixed counter 3 before write to
> PERF_METRICS.

Do we really have to support this trainwreck? This is such ill designed
hardware, I'm loath to support it, it might encourage more such
'creative' things and we really don't need that.