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

[Ingo Molnar <mingo@kernel.org>]

* 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?

Thanks,

	Ingo