selftests/x86/xstate: Clarify supported xstates

The established xstate test code is designed to be generic, but certain
xstates require special handling and cannot be tested without additional
adjustments.

Clarify which xstates are currently supported, and enforce testing only
for them.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250226010731.2456-9-chang.seok.bae@intel.com

tools/testing/selftests/x86/xstate.c

@@ -15,6 +15,24 @@
 #include "helpers.h"
 #include "xstate.h"
 
+/*
+ * The userspace xstate test suite is designed to be generic and operates
+ * with randomized xstate data. However, some states require special handling:
+ *
+ * - PKRU and XTILECFG need specific adjustments, such as modifying
+ *   randomization behavior or using fixed values.
+ * - But, PKRU already has a dedicated test suite in /tools/selftests/mm.
+ * - Legacy states (FP and SSE) are excluded, as they are not considered
+ *   part of extended states (xstates) and their usage is already deeply
+ *   integrated into user-space libraries.
+ */
+#define XFEATURE_MASK_TEST_SUPPORTED	\
+	((1 << XFEATURE_YMM) |		\
+	 (1 << XFEATURE_OPMASK)	|	\
+	 (1 << XFEATURE_ZMM_Hi256) |	\
+	 (1 << XFEATURE_Hi16_ZMM) |	\
+	 (1 << XFEATURE_XTILEDATA))
+
 static inline uint64_t xgetbv(uint32_t index)
 {
 	uint32_t eax, edx;
@@ -435,6 +453,12 @@ void test_xstate(uint32_t feature_num)
 	unsigned long features;
 	long rc;
 
+	if (!(XFEATURE_MASK_TEST_SUPPORTED & (1 << feature_num))) {
+		ksft_print_msg("The xstate test does not fully support the component %u, yet.\n",
+			       feature_num);
+		return;
+	}
+
 	rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
 	if (rc || !(features & (1 << feature_num))) {
 		ksft_print_msg("The kernel does not support feature number: %u\n", feature_num);