selftests/x86/avx: Add AVX tests

Add xstate testing specifically for those vector register states,
validating kernel's context switching and ensuring ABI compliance.
Use the established xstate testing framework.

Alternatively, this invocation could be placed directly in
xstate.c::main(). However, the current test file naming convention, which
clearly specifies the tested area, seems reasonable. Adding avx.c
considerably aligns with that convention.

The test output should be like this for ZMM_Hi256 as an example:

  $ avx_64
  ...
  [RUN]   AVX-512 ZMM_Hi256: check context switches, 10 iterations, 5 threads.
  [OK]    No incorrect case was found.
  [RUN]   AVX-512 ZMM_Hi256: inject xstate via ptrace().
  [OK]    'xfeatures' in SW reserved area was correctly written
  [OK]    xstate was correctly updated.
  [RUN]   AVX-512 ZMM_Hi256: load xstate and raise SIGUSR1
  [OK]    'magic1' is valid
  [OK]    'xfeatures' in SW reserved area is valid
  [OK]    'xfeatures' in XSAVE header is valid
  [OK]    xstate delivery was successful
  [OK]    'magic2' is valid
  [RUN]   AVX-512 ZMM_Hi256: load new xstate from sighandler and check it after sigreturn
  [OK]    xstate was restored correctly

But systems without AVX-512 will look like:
  ...
  The kernel does not support feature number: 5
  The kernel does not support feature number: 6
  The kernel does not support feature number: 7

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/20250226010731.2456-10-chang.seok.bae@intel.com

tools/testing/selftests/x86/Makefile

@@ -19,7 +19,7 @@ TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
 TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip syscall_numbering \
-			corrupt_xstate_header amx lam test_shadow_stack
+			corrupt_xstate_header amx lam test_shadow_stack avx
 # Some selftests require 32bit support enabled also on 64bit systems
 TARGETS_C_32BIT_NEEDED := ldt_gdt ptrace_syscall
 
@@ -133,4 +133,6 @@ $(OUTPUT)/check_initial_reg_state_64: CFLAGS += -Wl,-ereal_start -static
 $(OUTPUT)/nx_stack_32: CFLAGS += -Wl,-z,noexecstack
 $(OUTPUT)/nx_stack_64: CFLAGS += -Wl,-z,noexecstack
 
+$(OUTPUT)/avx_64: CFLAGS += -mno-avx -mno-avx512f
 $(OUTPUT)/amx_64: EXTRA_FILES += xstate.c
+$(OUTPUT)/avx_64: EXTRA_FILES += xstate.c

tools/testing/selftests/x86/avx.c

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE /* Required for inline xstate helpers */
+#include "xstate.h"
+
+int main(void)
+{
+	test_xstate(XFEATURE_YMM);
+	test_xstate(XFEATURE_OPMASK);
+	test_xstate(XFEATURE_ZMM_Hi256);
+	test_xstate(XFEATURE_Hi16_ZMM);
+}