linux-kernel - Re: [PATCH V2 1/8] perf/x86/uncore: Save the unit control address of all units

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Date: Mon, 17 Jun 2024 13:51:28 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...nel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
 namhyung@...nel.org, irogers@...gle.com, eranian@...gle.com,
 ak@...ux.intel.com, yunying.sun@...el.com, tim.c.chen@...ux.intel.com
Subject: Re: [PATCH V2 1/8] perf/x86/uncore: Save the unit control address of
 all units



On 2024-06-17 11:50 a.m., Peter Zijlstra wrote:
> On Fri, Jun 14, 2024 at 06:46:24AM -0700, kan.liang@...ux.intel.com wrote:
>> From: Kan Liang <kan.liang@...ux.intel.com>
>>
>> The unit control address of some CXL units may be wrongly calculated
>> under some configuration on a EMR machine.
>>
>> The current implementation only saves the unit control address of the
>> units from the first die, and the first unit of the rest of dies. Perf
>> assumed that the units from the other dies have the same offset as the
>> first die. So the unit control address of the rest of the units can be
>> calculated. However, the assumption is wrong, especially for the CXL
>> units.
> 
> Oh gawd, that's terrible. Was this actually specified, or are we hacking
> around a firmware fail?

Not a workaround for a firmware fail. The the CXL discovery units depend
on the stacks and CXL devices present on each socket. So different
number of CXL devices on each socket is a valid combination.
SW cannot calculate the address of the devices on the other sockets via
the first socket. Full addresses have to be used.

Thanks,
Kan