Re: [PATCH v5 2/2] selftests/resctrl: Recognise aarch64 as a vendor for L3_NONCONT_CAT

[Ben Horgan <ben.horgan@arm.com>]

Hi Richard,

On 12/10/2026 08:47, Richard Cheng wrote:
> aarch64 has no vendor_id in /proc/cpuinfo, so detect_vendor() returns 0
> and arch_supports_noncont_cat() falls through to "return false".
> L3_NONCONT_CAT therefore spuriously fails on every ARM MPAM platform.
> 
> Define ARCH_ARM, short-circuit detect_vendor() to it on aarch64, and
> add it to the AMD/Hygon always-supports early-out in
> arch_supports_noncont_cat().
> 
> aarch64 has many implementers (ARM 0x41, NVIDIA 0x43, etc.), but MPAM
> mandates non-contiguous CPBM uniformly, so per-implementer handling is
> not needed here.
> 
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
> Signed-off-by: Richard Cheng <icheng@nvidia.com>
> ---
>  tools/testing/selftests/resctrl/cat_test.c    |  9 ++++++--
>  tools/testing/selftests/resctrl/resctrl.h     |  1 +
>  .../testing/selftests/resctrl/resctrl_tests.c | 21 +++++++++++++++++++
>  3 files changed, 29 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> index 0a417267dd74..f4b02b3eedd7 100644
> --- a/tools/testing/selftests/resctrl/cat_test.c
> +++ b/tools/testing/selftests/resctrl/cat_test.c
> @@ -271,8 +271,13 @@ static bool arch_supports_noncont_cat(const struct resctrl_test *test)
>  {
>  	unsigned int vendor_id = get_vendor();
>  
> -	/* AMD and Hygon always support non-contiguous CBM. */
> -	if (vendor_id == ARCH_AMD || vendor_id == ARCH_HYGON)
> +	/*
> +	 * AMD and Hygon always support non-contiguous CBM. ARM/MPAM defines
> +	 * MPAMCFG_CPBM as a bitmap with no contiguity constraint per ARM
> +	 * DDI 0598.
> +	 */
> +	if (vendor_id == ARCH_AMD || vendor_id == ARCH_HYGON ||
> +	    vendor_id == ARCH_ARM)
>  		return true;

Would it be better not to just to use the information resctrl advertises?
/sys/fs/resctrl/info/L3/sparse_masks

Is there any reason this test doesn't use that other than being written before its introduction?

Thanks,

Ben

>  
>  #if defined(__i386__) || defined(__x86_64__) /* arch */
> diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h
> index a7556cdae0de..03b4500c1f7f 100644
> --- a/tools/testing/selftests/resctrl/resctrl.h
> +++ b/tools/testing/selftests/resctrl/resctrl.h
> @@ -41,6 +41,7 @@
>  #define ARCH_INTEL	BIT(0)
>  #define ARCH_AMD	BIT(1)
>  #define ARCH_HYGON	BIT(2)
> +#define ARCH_ARM	BIT(3)
>  
>  #define END_OF_TESTS	1
>  
> diff --git a/tools/testing/selftests/resctrl/resctrl_tests.c b/tools/testing/selftests/resctrl/resctrl_tests.c
> index dbcd5eea9fbc..cfece594a8c6 100644
> --- a/tools/testing/selftests/resctrl/resctrl_tests.c
> +++ b/tools/testing/selftests/resctrl/resctrl_tests.c
> @@ -23,6 +23,15 @@ static struct resctrl_test *resctrl_tests[] = {
>  	&l2_noncont_cat_test,
>  };
>  
> +static bool detect_aarch64(void)
> +{
> +#if defined(__aarch64__)
> +	return true;
> +#else
> +	return false;
> +#endif
> +}
> +
>  static unsigned int detect_vendor(void)
>  {
>  	static unsigned int vendor_id;
> @@ -34,6 +43,18 @@ static unsigned int detect_vendor(void)
>  	if (initialized)
>  		return vendor_id;
>  
> +	if (detect_aarch64()) {
> +		/*
> +		 * aarch64 has no userspace vendor_id in /proc/cpuinfo.
> +		 * MPAM-capable ARM implementations follow ARM DDI 0598;
> +		 * treat all aarch64 builds as a single vendor for the
> +		 * purposes of resctrl selftests.
> +		 */
> +		vendor_id = ARCH_ARM;
> +		initialized = true;
> +		return vendor_id;
> +	}
> +
>  	inf = fopen("/proc/cpuinfo", "r");
>  	if (!inf) {
>  		vendor_id = 0;