[PATCH v3 2/3] selftests/x86/amx: Fix the test to avoid failure when AMX is unavailable

[Chang S. Bae]

When a CPU does not have AMX, the test fails. But this is wrong as it
should be runnable regardless. Skip the test instead.

Also, simplify the feature check using arch_prctl() instead of CPUID. The
syscall is more trustworthy as the kernel controls the feature permission.

Reported-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Fixes: 6a3e0651b4a ("selftests/x86/amx: Add test cases for AMX state management")
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: linux-kselftest@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Changes from v2: None

Changes from v1:
* Simplify the feature check code (Shuah Khan).
---
 tools/testing/selftests/x86/amx.c | 31 ++++++++++---------------------
 1 file changed, 10 insertions(+), 21 deletions(-)

diff --git a/tools/testing/selftests/x86/amx.c b/tools/testing/selftests/x86/amx.c
index d884fd69dd51..f0b1efe89ef7 100644
--- a/tools/testing/selftests/x86/amx.c
+++ b/tools/testing/selftests/x86/amx.c
@@ -101,24 +101,6 @@ static void clearhandler(int sig)
 #define XFEATURE_MASK_XTILEDATA	(1 << XFEATURE_XTILEDATA)
 #define XFEATURE_MASK_XTILE	(XFEATURE_MASK_XTILECFG | XFEATURE_MASK_XTILEDATA)
 
-#define CPUID_LEAF1_ECX_XSAVE_MASK	(1 << 26)
-#define CPUID_LEAF1_ECX_OSXSAVE_MASK	(1 << 27)
-static inline void check_cpuid_xsave(void)
-{
-	uint32_t eax, ebx, ecx, edx;
-
-	/*
-	 * CPUID.1:ECX.XSAVE[bit 26] enumerates general
-	 * support for the XSAVE feature set, including
-	 * XGETBV.
-	 */
-	__cpuid_count(1, 0, eax, ebx, ecx, edx);
-	if (!(ecx & CPUID_LEAF1_ECX_XSAVE_MASK))
-		fatal_error("cpuid: no CPU xsave support");
-	if (!(ecx & CPUID_LEAF1_ECX_OSXSAVE_MASK))
-		fatal_error("cpuid: no OS xsave support");
-}
-
 static uint32_t xbuf_size;
 
 static struct {
@@ -350,6 +332,7 @@ enum expected_result { FAIL_EXPECTED, SUCCESS_EXPECTED };
 
 /* arch_prctl() and sigaltstack() test */
 
+#define ARCH_GET_XCOMP_SUPP	0x1021
 #define ARCH_GET_XCOMP_PERM	0x1022
 #define ARCH_REQ_XCOMP_PERM	0x1023
 
@@ -928,10 +911,16 @@ static void test_ptrace(void)
 
 int main(void)
 {
-	/* Check hardware availability at first */
-	check_cpuid_xsave();
-	check_cpuid_xtiledata();
+	unsigned long features;
+	long rc;
 
+	rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
+	if (rc || (features & XFEATURE_MASK_XTILE) != XFEATURE_MASK_XTILE) {
+		printf("[SKIP]\tno AMX support.\n");
+		exit(KSFT_FAIL);
+	}
+
+	check_cpuid_xtiledata();
 	init_stashed_xsave();
 	sethandler(SIGILL, handle_noperm, 0);
 
-- 
2.17.1