Re: [PATCH] perf/x86/uncore: Correct the number of CHAs on EMR

[Ingo Molnar <mingo@kernel.org>]

* Liang, Kan <kan.liang@linux.intel.com> wrote:

> 
> 
> On 2023-09-03 4:40 a.m., Ingo Molnar wrote:
> > 
> > * kan.liang@linux.intel.com <kan.liang@linux.intel.com> wrote:
> > 
> >> From: Kan Liang <kan.liang@linux.intel.com>
> >>
> >> The MSR UNC_CBO_CONFIG, which was used to detect the number of CHAs on
> >> SPR, is broken on EMR XCC. It always returns 0.
> >>
> >> Roll back to the discovery method, which can give the correct number for
> >> this case.
> >>
> >> Fixes: 38776cc45eb7 ("perf/x86/uncore: Correct the number of CHAs on SPR")
> >> Reported-by: Stephane Eranian <eranian@google.com>
> >> Reported-by: Yunying Sun <yunying.sun@intel.com>
> >> Tested-by: Yunying Sun <yunying.sun@intel.com>
> >> Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
> >> ---
> >>  arch/x86/events/intel/uncore_snbep.c | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/x86/events/intel/uncore_snbep.c b/arch/x86/events/intel/uncore_snbep.c
> >> index d49e90dc04a4..c41d7d46481c 100644
> >> --- a/arch/x86/events/intel/uncore_snbep.c
> >> +++ b/arch/x86/events/intel/uncore_snbep.c
> >> @@ -6475,7 +6475,9 @@ void spr_uncore_cpu_init(void)
> >>  	type = uncore_find_type_by_id(uncore_msr_uncores, UNCORE_SPR_CHA);
> >>  	if (type) {
> >>  		rdmsrl(SPR_MSR_UNC_CBO_CONFIG, num_cbo);
> >> -		type->num_boxes = num_cbo;
> >> +		/* The MSR doesn't work on the EMR XCC. Roll back to the discovery method. */
> >> +		if (num_cbo)
> >> +			type->num_boxes = num_cbo;
> > 
> > So in the zero case we don't write type->num_boxes and leave it as-is.
> > 
> > How does this fall back to the discovery method, is the existing (default?) 
> > value of type->num_boxes some special value?
> > 
> 
> Starts from SPR, the basic uncore PMON information are retrieved from
> the discovery table (resides in an MMIO space populated by BIOS.). It is
> called the discovery method. The existing value of the type->num_boxes
> is from the discovery table.
> 
> On some SPR variants, there is a firmware bug. So the value from the
> discovery table is incorrect. We use the value from
> SPR_MSR_UNC_CBO_CONFIG to replace the one from the discovery table.
> 38776cc45eb7 ("perf/x86/uncore: Correct the number of CHAs on SPR")
> 
> Unfortunately, the SPR_MSR_UNC_CBO_CONFIG isn't available for the EMR
> XCC (It works well for other EMR variants). But the above firmware bug
> doesn't impact the EMR XCC. So this patch NOT lets the value from the
> SPR_MSR_UNC_CBO_CONFIG replace the existing value from the discovery table.

Thanks - the comment & changelog should probably reflect this background.

	Ingo