linux-kernel - Re: [PATCH] selftests: x86: skip the tests if prerequisites aren't fulfilled

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Date: Tue, 12 Mar 2024 09:07:15 -0700
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: Muhammad Usama Anjum <usama.anjum@...labora.com>, Shuah Khan
	<shuah@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>, "Kirill A.
 Shutemov" <kirill.shutemov@...ux.intel.com>, "Peter Zijlstra (Intel)"
	<peterz@...radead.org>, Weihong Zhang <weihong.zhang@...el.com>, Binbin Wu
	<binbin.wu@...ux.intel.com>, angquan yu <angquan21@...il.com>
CC: <kernel@...labora.com>, <linux-kselftest@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] selftests: x86: skip the tests if prerequisites aren't
 fulfilled

On 3/12/2024 2:26 AM, Muhammad Usama Anjum wrote:
 >
> How can we check if AMX is available or not?

After a successful check_cpuid_xsave(), examining CPUID(eax=0xd, ecx=0) 
EDX: EAX, which reports the support bits of XCR0, can give assurance of 
AMX availability. Perhaps, this change is considerable on top of your patch:

static int check_cpuid_xtiledata(void)
{
         uint32_t eax, ebx, ecx, edx;
+       uint64_t xfeatures;

         __cpuid_count(CPUID_LEAF_XSTATE, CPUID_SUBLEAF_XSTATE_USER,
                       eax, ebx, ecx, edx);

+       xfeatures = eax + ((uint64_t)edx << 32);
+       if ((xfeatures & XFEATURE_MASK_XTILE) != XFEATURE_MASK_XTILE) {
+               ksft_print_msg("no AMX support\n");
+               return -1;
+       }

Nevertheless, I still believe that using arch_prctl(ARCH_GET_XCOMP_SUPP, 
..) remains a simple and legitimate approach for directly checking 
dynamic feature support from the kernel: 
https://docs.kernel.org/arch/x86/xstate.html

Thanks,
Chang