Date:   Tue, 26 Oct 2021 05:25:24 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     dave.hansen@...el.com
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        chang.seok.bae@...el.com, tglx@...utronix.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: [PATCH 1/2] selftest/x86/amx: Test cases for AMX state management


From: Chang S. Bae <chang.seok.bae@...el.com>

AMX TILEDATA is a very large XSAVE feature.  It could have caused
nasty space XSAVE buffer space waste in two places:
 * Signal stacks
 * Kernel task_struct->fpu buffers

To avoid this waste, neither of these buffers have AMX state by
default.  The non-default features are called "dynamic" features.

There is an arch_prctl(ARCH_REQ_XCOMP_PERM) which allows a task
to declare that it wants to use AMX or other "dynamic" XSAVE
features.  This arch_prctl() ensures that sufficient sigaltstack
space is available before it will succeed.  It also expands the
task_struct buffer.

Functions of this test:
 * Test arch_prctl(ARCH_REQ_XCOMP_PERM).  Ensure that it checks for
   proper sigaltstack sizing and that the sizing is enforced for
   future sigaltstack calls.
 * Ensure that ARCH_REQ_XCOMP_PERM is inherited across fork()
 * Ensure that TILEDATA use before the prctl() is fatal
 * Ensure that TILEDATA is cleared across fork()

Note: Generally, compiler support is needed to do something with
AMX.  Instead, directly load AMX state from userspace with a
plain XSAVE.  Do not depend on the compiler.

 [ dhansen: bunches of cleanups ]

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
Cc: linux-kselftest@...r.kernel.org

---

 b/tools/testing/selftests/x86/Makefile |    2 
 b/tools/testing/selftests/x86/amx.c    |  697 +++++++++++++++++++++++++++++++++
 2 files changed, 698 insertions(+), 1 deletion(-)

diff -puN /dev/null tools/testing/selftests/x86/amx.c
--- /dev/null	2021-10-02 19:44:55.430567491 -0700
+++ b/tools/testing/selftests/x86/amx.c	2021-10-15 13:56:39.829901810 -0700
@@ -0,0 +1,697 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#define _GNU_SOURCE
+#include <err.h>
+#include <errno.h>
+#include <setjmp.h>
+#include <stdio.h>
+#include <string.h>
+#include <stdbool.h>
+#include <unistd.h>
+#include <x86intrin.h>
+
+#include <linux/futex.h>
+
+#include <sys/auxv.h>
+#include <sys/mman.h>
+#include <sys/shm.h>
+#include <sys/syscall.h>
+#include <sys/wait.h>
+
+#ifndef __x86_64__
+# error This test is 64-bit only
+#endif
+
+#define XSAVE_HDR_OFFSET	512
+#define XSAVE_HDR_SIZE		64
+
+struct xsave_buffer {
+	union {
+		struct {
+			char legacy[XSAVE_HDR_OFFSET];
+			char header[XSAVE_HDR_SIZE];
+			char extended[0];
+		};
+		char bytes[0];
+	};
+};
+
+static inline uint64_t xgetbv(uint32_t index)
+{
+	uint32_t eax, edx;
+
+	asm volatile("xgetbv;"
+		     : "=a" (eax), "=d" (edx)
+		     : "c" (index));
+	return eax + ((uint64_t)edx << 32);
+}
+
+static inline void cpuid(uint32_t *eax, uint32_t *ebx, uint32_t *ecx, uint32_t *edx)
+{
+	asm volatile("cpuid;"
+		     : "=a" (*eax), "=b" (*ebx), "=c" (*ecx), "=d" (*edx)
+		     : "0" (*eax), "2" (*ecx));
+}
+
+static inline void xsave(struct xsave_buffer *xbuf, uint64_t rfbm)
+{
+	uint32_t rfbm_lo = rfbm;
+	uint32_t rfbm_hi = rfbm >> 32;
+
+	asm volatile("xsave (%%rdi)"
+		     : : "D" (xbuf), "a" (rfbm_lo), "d" (rfbm_hi)
+		     : "memory");
+}
+
+static inline void xrstor(struct xsave_buffer *xbuf, uint64_t rfbm)
+{
+	uint32_t rfbm_lo = rfbm;
+	uint32_t rfbm_hi = rfbm >> 32;
+
+	asm volatile("xrstor (%%rdi)"
+		     : : "D" (xbuf), "a" (rfbm_lo), "d" (rfbm_hi));
+}
+
+/* err() exits and will not return */
+#define fatal_error(msg, ...)	err(1, "[FAIL]\t" msg, ##__VA_ARGS__)
+
+static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
+		       int flags)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_sigaction = handler;
+	sa.sa_flags = SA_SIGINFO | flags;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		fatal_error("sigaction");
+}
+
+static void clearhandler(int sig)
+{
+	struct sigaction sa;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_handler = SIG_DFL;
+	sigemptyset(&sa.sa_mask);
+	if (sigaction(sig, &sa, 0))
+		fatal_error("sigaction");
+}
+
+#define XFEATURE_XTILECFG	17
+#define XFEATURE_XTILEDATA	18
+#define XFEATURE_MASK_XTILECFG	(1 << XFEATURE_XTILECFG)
+#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.
+	 */
+	eax = 1;
+	ecx = 0;
+	cpuid(&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 {
+	uint32_t xbuf_offset;
+	uint32_t size;
+} xtiledata;
+
+#define CPUID_LEAF_XSTATE		0xd
+#define CPUID_SUBLEAF_XSTATE_USER	0x0
+#define TILE_CPUID			0x1d
+#define TILE_PALETTE_ID			0x1
+
+static void check_cpuid_xtiledata(void)
+{
+	uint32_t eax, ebx, ecx, edx;
+
+	eax = CPUID_LEAF_XSTATE;
+	ecx = CPUID_SUBLEAF_XSTATE_USER;
+	cpuid(&eax, &ebx, &ecx, &edx);
+
+	/*
+	 * EBX enumerates the size (in bytes) required by the XSAVE
+	 * instruction for an XSAVE area containing all the user state
+	 * components corresponding to bits currently set in XCR0.
+	 *
+	 * Stash that off so it can be used to allocate buffers later.
+	 */
+	xbuf_size = ebx;
+
+	eax = CPUID_LEAF_XSTATE;
+	ecx = XFEATURE_XTILEDATA;
+
+	cpuid(&eax, &ebx, &ecx, &edx);
+	/*
+	 * eax: XTILEDATA state component size
+	 * ebx: XTILEDATA state component offset in user buffer
+	 */
+	if (!eax || !ebx)
+		fatal_error("xstate cpuid: invalid tile data size/offset: %d/%d",
+				eax, ebx);
+
+	xtiledata.size	      = eax;
+	xtiledata.xbuf_offset = ebx;
+}
+
+/* The helpers for managing XSAVE buffer and tile states: */
+
+struct xsave_buffer *alloc_xbuf(void)
+{
+	struct xsave_buffer *xbuf;
+
+	/* XSAVE buffer should be 64B-aligned. */
+	xbuf = aligned_alloc(64, xbuf_size);
+	if (!xbuf)
+		fatal_error("aligned_alloc()");
+	return xbuf;
+}
+
+static inline void clear_xstate_header(struct xsave_buffer *buffer)
+{
+	memset(&buffer->header, 0, sizeof(buffer->header));
+}
+
+static inline uint64_t get_xstatebv(struct xsave_buffer *buffer)
+{
+	/* XSTATE_BV is at the beginning of the header: */
+	return *(uint64_t *)&buffer->header;
+}
+
+static inline void set_xstatebv(struct xsave_buffer *buffer, uint64_t bv)
+{
+	/* XSTATE_BV is at the beginning of the header: */
+	*(uint64_t *)(&buffer->header) = bv;
+}
+
+static void set_rand_tiledata(struct xsave_buffer *xbuf)
+{
+	int *ptr = (int *)&xbuf->bytes[xtiledata.xbuf_offset];
+	int data;
+	int i;
+
+	/*
+	 * Ensure that 'data' is never 0.  This ensures that
+	 * the registers are never in their initial configuration
+	 * and thus never tracked as being in the init state.
+	 */
+	data = rand() | 1;
+
+	for (i = 0; i < xtiledata.size / sizeof(int); i++, ptr++)
+		*ptr = data;
+}
+
+struct xsave_buffer *stashed_xsave;
+
+static void init_stashed_xsave(void)
+{
+	stashed_xsave = alloc_xbuf();
+	if (!stashed_xsave)
+		fatal_error("failed to allocate stashed_xsave\n");
+	clear_xstate_header(stashed_xsave);
+}
+
+static void free_stashed_xsave(void)
+{
+	free(stashed_xsave);
+}
+
+/* See 'struct _fpx_sw_bytes' at sigcontext.h */
+#define SW_BYTES_OFFSET		464
+/* N.B. The struct's field name varies so read from the offset. */
+#define SW_BYTES_BV_OFFSET	(SW_BYTES_OFFSET + 8)
+
+static inline struct _fpx_sw_bytes *get_fpx_sw_bytes(void *buffer)
+{
+	return (struct _fpx_sw_bytes *)(buffer + SW_BYTES_OFFSET);
+}
+
+static inline uint64_t get_fpx_sw_bytes_features(void *buffer)
+{
+	return *(uint64_t *)(buffer + SW_BYTES_BV_OFFSET);
+}
+
+/* Work around printf() being unsafe in signals: */
+#define SIGNAL_BUF_LEN 1000
+char signal_message_buffer[SIGNAL_BUF_LEN];
+void sig_print(char *msg)
+{
+	int left = SIGNAL_BUF_LEN - strlen(signal_message_buffer) - 1;
+
+	strncat(signal_message_buffer, msg, left);
+}
+
+static volatile bool noperm_signaled;
+static int noperm_errs;
+
+/*
+ * Signal handler for when AMX is used but
+ * permission has not been obtained.
+ */
+static void handle_noperm(int sig, siginfo_t *si, void *ctx_void)
+{
+	ucontext_t *ctx = (ucontext_t *)ctx_void;
+	void *xbuf = ctx->uc_mcontext.fpregs;
+	struct _fpx_sw_bytes *sw_bytes;
+	uint64_t features;
+
+	/* Reset the signal message buffer: */
+	signal_message_buffer[0] = '\0';
+	sig_print("\tAt SIGILL handler,\n");
+
+	if (si->si_code != ILL_ILLOPC) {
+		noperm_errs++;
+		sig_print("[FAIL]\tInvalid signal code.\n");
+	} else {
+		sig_print("[OK]\tValid signal code (ILL_ILLOPC).\n");
+	}
+
+	sw_bytes = get_fpx_sw_bytes(xbuf);
+	/*
+	 * Without permission, the signal XSAVE buffer should not
+	 * have room for AMX register state (aka. xtiledata).
+	 * Check that the size does not overlap with where xtiledata
+	 * will reside.
+	 *
+	 * This also implies that no state components *PAST*
+	 * XTILEDATA (features >=19) can be present in the buffer.
+	 */
+	if (sw_bytes->xstate_size <= xtiledata.xbuf_offset) {
+		sig_print("[OK]\tValid xstate size\n");
+	} else {
+		noperm_errs++;
+		sig_print("[FAIL]\tInvalid xstate size\n");
+	}
+
+	features = get_fpx_sw_bytes_features(xbuf);
+	/*
+	 * Without permission, the XTILEDATA feature
+	 * bit should not be set.
+	 */
+	if ((features & XFEATURE_MASK_XTILEDATA) == 0) {
+		sig_print("[OK]\tValid xstate mask\n");
+	} else {
+		noperm_errs++;
+		sig_print("[FAIL]\tInvalid xstate mask\n");
+	}
+
+	noperm_signaled = true;
+	ctx->uc_mcontext.gregs[REG_RIP] += 3; /* Skip the faulting XRSTOR */
+}
+
+/* Return true if XRSTOR is successful; otherwise, false. */
+static inline bool xrstor_safe(struct xsave_buffer *xbuf, uint64_t mask)
+{
+	noperm_signaled = false;
+	xrstor(xbuf, mask);
+
+	/* Print any messages produced by the signal code: */
+	printf("%s", signal_message_buffer);
+	/*
+	 * Reset the buffer to make sure any future printing
+	 * only outputs new messages:
+	 */
+	signal_message_buffer[0] = '\0';
+
+	if (noperm_errs)
+		fatal_error("saw %d errors in noperm signal handler\n", noperm_errs);
+
+	return !noperm_signaled;
+}
+
+/*
+ * Use XRSTOR to populate the XTILEDATA registers with
+ * random data.
+ *
+ * Return true if successful; otherwise, false.
+ */
+static inline bool load_rand_tiledata(struct xsave_buffer *xbuf)
+{
+	clear_xstate_header(xbuf);
+	set_xstatebv(xbuf, XFEATURE_MASK_XTILEDATA);
+	set_rand_tiledata(xbuf);
+	return xrstor_safe(xbuf, XFEATURE_MASK_XTILEDATA);
+}
+
+/* Return XTILEDATA to its initial configuration. */
+static inline void init_xtiledata(void)
+{
+	clear_xstate_header(stashed_xsave);
+	xrstor_safe(stashed_xsave, XFEATURE_MASK_XTILEDATA);
+}
+
+enum expected_result { FAIL_EXPECTED, SUCCESS_EXPECTED };
+
+/* arch_prctl() and sigaltstack() test */
+
+#define ARCH_GET_XCOMP_PERM	0x1022
+#define ARCH_REQ_XCOMP_PERM	0x1023
+
+static void req_xtiledata_perm(void)
+{
+	syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, XFEATURE_XTILEDATA);
+}
+
+static void validate_req_xcomp_perm(enum expected_result exp)
+{
+	unsigned long bitmask;
+	long rc;
+
+	rc = syscall(SYS_arch_prctl, ARCH_REQ_XCOMP_PERM, XFEATURE_XTILEDATA);
+	if (exp == FAIL_EXPECTED) {
+		if (rc) {
+			printf("[OK]\tARCH_REQ_XCOMP_PERM saw expected failure..\n");
+			return;
+		}
+
+		fatal_error("ARCH_REQ_XCOMP_PERM saw unexpected success.\n");
+	} else if (rc) {
+		fatal_error("ARCH_REQ_XCOMP_PERM saw unexpected failure.\n");
+	}
+
+	rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_PERM, &bitmask);
+	if (rc) {
+		fatal_error("prctl(ARCH_GET_XCOMP_PERM) error: %ld", rc);
+	} else if (bitmask & XFEATURE_MASK_XTILE) {
+		printf("\tARCH_REQ_XCOMP_PERM is successful.\n");
+	}
+}
+
+static void validate_xcomp_perm(enum expected_result exp)
+{
+	bool load_success = load_rand_tiledata(stashed_xsave);
+
+	if (exp == FAIL_EXPECTED) {
+		if (load_success) {
+			noperm_errs++;
+			printf("[FAIL]\tLoad tiledata succeeded.\n");
+		} else {
+			printf("[OK]\tLoad tiledata failed.\n");
+		}
+	} else if (exp == SUCCESS_EXPECTED) {
+		if (load_success) {
+			printf("[OK]\tLoad tiledata succeeded.\n");
+		} else {
+			noperm_errs++;
+			printf("[FAIL]\tLoad tiledata failed.\n");
+		}
+	}
+}
+
+#ifndef AT_MINSIGSTKSZ
+#  define AT_MINSIGSTKSZ	51
+#endif
+
+static void *alloc_altstack(unsigned int size)
+{
+	void *altstack;
+
+	altstack = mmap(NULL, size, PROT_READ | PROT_WRITE,
+			MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
+
+	if (altstack == MAP_FAILED)
+		fatal_error("mmap() for altstack");
+
+	return altstack;
+}
+
+static void setup_altstack(void *addr, unsigned long size, enum expected_result exp)
+{
+	stack_t ss;
+	int rc;
+
+	memset(&ss, 0, sizeof(ss));
+	ss.ss_size = size;
+	ss.ss_sp = addr;
+
+	rc = sigaltstack(&ss, NULL);
+
+	if (exp == FAIL_EXPECTED) {
+		if (rc) {
+			printf("[OK]\tsigaltstack() failed.\n");
+		} else {
+			fatal_error("sigaltstack() succeeded unexpectedly.\n");
+		}
+	} else if (rc) {
+		fatal_error("sigaltstack()");
+	}
+}
+
+static void test_dynamic_sigaltstack(void)
+{
+	unsigned int small_size, enough_size;
+	unsigned long minsigstksz;
+	void *altstack;
+
+	minsigstksz = getauxval(AT_MINSIGSTKSZ);
+	printf("\tAT_MINSIGSTKSZ = %lu\n", minsigstksz);
+	/*
+	 * getauxval() itself can return 0 for failure or
+	 * success.  But, in this case, AT_MINSIGSTKSZ
+	 * will always return a >=0 value if implemented.
+	 * Just check for 0.
+	 */
+	if (minsigstksz == 0) {
+		printf("no support for AT_MINSIGSTKSZ, skipping sigaltstack tests\n");
+		return;
+	}
+
+	enough_size = minsigstksz * 2;
+
+	altstack = alloc_altstack(enough_size);
+	printf("\tAllocate memory for altstack (%u bytes).\n", enough_size);
+
+	/*
+	 * Try setup_altstack() with a size which can not fit
+	 * XTILEDATA.  ARCH_REQ_XCOMP_PERM should fail.
+	 */
+	small_size = minsigstksz - xtiledata.size;
+	printf("\tAfter sigaltstack() with small size (%u bytes).\n", small_size);
+	setup_altstack(altstack, small_size, SUCCESS_EXPECTED);
+	validate_req_xcomp_perm(FAIL_EXPECTED);
+
+	/*
+	 * Try setup_altstack() with a size derived from
+	 * AT_MINSIGSTKSZ.  It should be more than large enough
+	 * and thus ARCH_REQ_XCOMP_PERM should succeed.
+	 */
+	printf("\tAfter sigaltstack() with enough size (%u bytes).\n", enough_size);
+	setup_altstack(altstack, enough_size, SUCCESS_EXPECTED);
+	validate_req_xcomp_perm(SUCCESS_EXPECTED);
+
+	/*
+	 * Try to coerce setup_altstack() to again accept a
+	 * too-small altstack.  This ensures that big-enough
+	 * sigaltstacks can not shrink to a too-small value
+	 * once XTILEDATA permission is established.
+	 */
+	printf("\tThen, sigaltstack() with small size (%u bytes).\n", small_size);
+	setup_altstack(altstack, small_size, FAIL_EXPECTED);
+}
+
+static void test_dynamic_state(void)
+{
+	pid_t parent, child, grandchild;
+
+	parent = fork();
+	if (parent < 0) {
+		/* fork() failed */
+		fatal_error("fork");
+	} else if (parent > 0) {
+		int status;
+		/* fork() succeeded.  Now in the parent. */
+
+		wait(&status);
+		if (!WIFEXITED(status) || WEXITSTATUS(status))
+			fatal_error("arch_prctl test parent exit");
+		return;
+	}
+	/* fork() succeeded.  Now in the child . */
+
+	printf("[RUN]\tCheck ARCH_REQ_XCOMP_PERM around process fork() and sigaltack() test.\n");
+
+	printf("\tFork a child.\n");
+	child = fork();
+	if (child < 0) {
+		fatal_error("fork");
+	} else if (child > 0) {
+		int status;
+
+		wait(&status);
+		if (!WIFEXITED(status) || WEXITSTATUS(status))
+			fatal_error("arch_prctl test child exit");
+		_exit(0);
+	}
+
+	/*
+	 * The permission request should fail without an
+	 * XTILEDATA-compatible signal stack
+	 */
+	printf("\tTest XCOMP_PERM at child.\n");
+	validate_xcomp_perm(FAIL_EXPECTED);
+
+	/*
+	 * Set up an XTILEDATA-compatible signal stack and
+	 * also obtain permission to populate XTILEDATA.
+	 */
+	printf("\tTest dynamic sigaltstack at child:\n");
+	test_dynamic_sigaltstack();
+
+	/* Ensure that XTILEDATA can be populated. */
+	printf("\tTest XCOMP_PERM again at child.\n");
+	validate_xcomp_perm(SUCCESS_EXPECTED);
+
+	printf("\tFork a grandchild.\n");
+	grandchild = fork();
+	if (grandchild < 0) {
+		/* fork() failed */
+		fatal_error("fork");
+	} else if (!grandchild) {
+		/* fork() succeeded.  Now in the (grand)child. */
+		printf("\tTest XCOMP_PERM at grandchild.\n");
+
+		/*
+		 * Ensure that the grandchild inherited
+		 * permission and a compatible sigaltstack:
+		 */
+		validate_xcomp_perm(SUCCESS_EXPECTED);
+	} else {
+		int status;
+		/* fork() succeeded.  Now in the parent. */
+
+		wait(&status);
+		if (!WIFEXITED(status) || WEXITSTATUS(status))
+			fatal_error("fork test grandchild");
+	}
+
+	_exit(0);
+}
+
+/*
+ * Save current register state and compare it to @xbuf1.'
+ *
+ * Returns false if @xbuf1 matches the registers.
+ * Returns true  if @xbuf1 differs from the registers.
+ */
+static inline bool __validate_tiledata_regs(struct xsave_buffer *xbuf1)
+{
+	struct xsave_buffer *xbuf2;
+	int ret;
+
+	xbuf2 = alloc_xbuf();
+	if (!xbuf2)
+		fatal_error("failed to allocate XSAVE buffer\n");
+
+	xsave(xbuf2, XFEATURE_MASK_XTILEDATA);
+	ret = memcmp(&xbuf1->bytes[xtiledata.xbuf_offset],
+		     &xbuf2->bytes[xtiledata.xbuf_offset],
+		     xtiledata.size);
+
+	free(xbuf2);
+
+	if (ret == 0)
+		return false;
+	return true;
+}
+
+static inline void validate_tiledata_regs_same(struct xsave_buffer *xbuf)
+{
+	int ret = __validate_tiledata_regs(xbuf);
+
+	if (ret != 0)
+		fatal_error("TILEDATA registers changed");
+}
+
+static inline void validate_tiledata_regs_changed(struct xsave_buffer *xbuf)
+{
+	int ret = __validate_tiledata_regs(xbuf);
+
+	if (ret == 0)
+		fatal_error("TILEDATA registers did not change");
+}
+
+/* tiledata inheritance test */
+
+static void test_fork(void)
+{
+	pid_t child, grandchild;
+
+	child = fork();
+	if (child < 0) {
+		/* fork() failed */
+		fatal_error("fork");
+	} else if (child > 0) {
+		/* fork() succeeded.  Now in the parent. */
+		int status;
+
+		wait(&status);
+		if (!WIFEXITED(status) || WEXITSTATUS(status))
+			fatal_error("fork test child");
+		return;
+	}
+	/* fork() succeeded.  Now in the child. */
+	printf("[RUN]\tCheck tile data inheritance.\n\tBefore fork(), load tiledata\n");
+
+	load_rand_tiledata(stashed_xsave);
+
+	grandchild = fork();
+	if (grandchild < 0) {
+		/* fork() failed */
+		fatal_error("fork");
+	} else if (grandchild > 0) {
+		/* fork() succeeded.  Still in the first child. */
+		int status;
+
+		wait(&status);
+		if (!WIFEXITED(status) || WEXITSTATUS(status))
+			fatal_error("fork test grand child");
+		_exit(0);
+	}
+	/* fork() succeeded.  Now in the (grand)child. */
+
+	/*
+	 * TILEDATA registers are not preserved across fork().
+	 * Ensure that their value has changed:
+	 */
+	validate_tiledata_regs_changed(stashed_xsave);
+
+	_exit(0);
+}
+
+int main(void)
+{
+	/* Check hardware availability at first */
+	check_cpuid_xsave();
+	check_cpuid_xtiledata();
+
+	init_stashed_xsave();
+	sethandler(SIGILL, handle_noperm, 0);
+
+	test_dynamic_state();
+
+	/* Request permission for the following tests */
+	req_xtiledata_perm();
+
+	test_fork();
+
+	clearhandler(SIGILL);
+	free_stashed_xsave();
+
+	return 0;
+}
diff -puN tools/testing/selftests/x86/Makefile~amx-selftest-1_3-selftest-x86-amx-Test-cases-for-the-AMX-state-management tools/testing/selftests/x86/Makefile
--- a/tools/testing/selftests/x86/Makefile~amx-selftest-1_3-selftest-x86-amx-Test-cases-for-the-AMX-state-management	2021-10-15 13:56:39.829901810 -0700
+++ b/tools/testing/selftests/x86/Makefile	2021-10-15 13:56:39.829901810 -0700
@@ -18,7 +18,7 @@ TARGETS_C_32BIT_ONLY := entry_from_vm86
 			test_FCMOV test_FCOMI test_FISTTP \
 			vdso_restorer
 TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip syscall_numbering \
-			corrupt_xstate_header
+			corrupt_xstate_header amx
 # Some selftests require 32bit support enabled also on 64bit systems
 TARGETS_C_32BIT_NEEDED := ldt_gdt ptrace_syscall
 
_

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