linux-kernel - Re: [PATCH 1/5] perf/x86/intel/uncore: Parse uncore discovery tables

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Date:   Tue, 16 Mar 2021 15:05:13 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     mingo@...nel.org, acme@...hat.com, linux-kernel@...r.kernel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        eranian@...gle.com, namhyung@...nel.org, ak@...ux.intel.com
Subject: Re: [PATCH 1/5] perf/x86/intel/uncore: Parse uncore discovery tables

On Tue, Mar 16, 2021 at 08:42:25AM -0400, Liang, Kan wrote:
> 
> 
> On 3/16/2021 7:43 AM, Peter Zijlstra wrote:
> > On Fri, Mar 12, 2021 at 08:34:34AM -0800, kan.liang@...ux.intel.com wrote:
> > > From: Kan Liang <kan.liang@...ux.intel.com>
> > > 
> > > A self-describing mechanism for the uncore PerfMon hardware has been
> > > introduced with the latest Intel platforms. By reading through an MMIO
> > > page worth of information, perf can 'discover' all the standard uncore
> > > PerfMon registers in a machine.
> > > 
> > > The discovery mechanism relies on BIOS's support. With a proper BIOS,
> > > a PCI device with the unique capability ID 0x23 can be found on each
> > > die. Perf can retrieve the information of all available uncore PerfMons
> > > from the device via MMIO. The information is composed of one global
> > > discovery table and several unit discovery tables.
> > 
> > > If a BIOS doesn't support the 'discovery' mechanism, there is nothing
> > > changed.
> > 
> > What if the BIOS got it wrong? Will the driver still get it correct if
> > it is a known platform?
> 
> Yes, I will submit a platform specific patch to fix this case.
> 
> > 
> > Do we need a chicken flag to kill the discovery? uncore_no_discover?
> > 
> 
> Yes, I plan to introduce a .use_discovery_tables flag to indicate whether to
> use the discovery tables for the known platform.
> 
> The below codes is part of the upcoming SPR uncore patches.
> The first SPR uncore patch will still rely on the BIOS discovery tables,
> because some uncore block information hasn't been published yet. We have to
> retrieve the information fro the tables. Once all the information is
> published, we can kill the discovery by removing the ".use_discovery_tables
> = true".

I was thinking of a module parameter, such that we can tell it to skip
discovery on module load time etc.