KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

4 files changed, 118 insertions, 0 deletions

diff --git a/tools/testing/selftests/fpu/.gitignore b/tools/testing/selftests/fpu/.gitignore
new file mode 100644
index 000000000..d6d12ac1d
--- /dev/null
+++ b/tools/testing/selftests/fpu/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0+
+test_fpu
diff --git a/tools/testing/selftests/fpu/Makefile b/tools/testing/selftests/fpu/Makefile
new file mode 100644
index 000000000..ea62c176e
--- /dev/null
+++ b/tools/testing/selftests/fpu/Makefile
@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-2.0+
+
+LDLIBS := -lm
+
+TEST_GEN_PROGS := test_fpu
+
+TEST_PROGS := run_test_fpu.sh
+
+include ../lib.mk
diff --git a/tools/testing/selftests/fpu/run_test_fpu.sh b/tools/testing/selftests/fpu/run_test_fpu.sh
new file mode 100755
index 000000000..d77be93ec
--- /dev/null
+++ b/tools/testing/selftests/fpu/run_test_fpu.sh
@@ -0,0 +1,46 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Load kernel module for FPU tests
+
+uid=$(id -u)
+if [ $uid -ne 0 ]; then
+	echo "$0: Must be run as root"
+	exit 1
+fi
+
+if ! which modprobe > /dev/null 2>&1; then
+	echo "$0: You need modprobe installed"
+        exit 4
+fi
+
+if ! modinfo test_fpu > /dev/null 2>&1; then
+	echo "$0: You must have the following enabled in your kernel:"
+	echo "CONFIG_TEST_FPU=m"
+	exit 4
+fi
+
+NR_CPUS=$(getconf _NPROCESSORS_ONLN)
+if [ ! $NR_CPUS ]; then
+	NR_CPUS=1
+fi
+
+modprobe test_fpu
+
+if [ ! -e /sys/kernel/debug/selftest_helpers/test_fpu ]; then
+	mount -t debugfs none /sys/kernel/debug
+
+	if [ ! -e /sys/kernel/debug/selftest_helpers/test_fpu ]; then
+		echo "$0: Error mounting debugfs"
+		exit 4
+	fi
+fi
+
+echo "Running 1000 iterations on all CPUs... "
+for i in $(seq 1 1000); do
+	for c in $(seq 1 $NR_CPUS); do
+		./test_fpu &
+	done
+done
+
+rmmod test_fpu
diff --git a/tools/testing/selftests/fpu/test_fpu.c b/tools/testing/selftests/fpu/test_fpu.c
new file mode 100644
index 000000000..200238522
--- /dev/null
+++ b/tools/testing/selftests/fpu/test_fpu.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0+
+/* This testcase operates with the test_fpu kernel driver.
+ * It modifies the FPU control register in user mode and calls the kernel
+ * module to perform floating point operations in the kernel. The control
+ * register value should be independent between kernel and user mode.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include <fenv.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+const char *test_fpu_path = "/sys/kernel/debug/selftest_helpers/test_fpu";
+
+int main(void)
+{
+	char dummy[1];
+	int fd = open(test_fpu_path, O_RDONLY);
+
+	if (fd < 0) {
+		printf("[SKIP]\tcan't access %s: %s\n",
+		       test_fpu_path, strerror(errno));
+		return 0;
+	}
+
+	if (read(fd, dummy, 1) < 0) {
+		printf("[FAIL]\taccess with default rounding mode failed\n");
+		return 1;
+	}
+
+	fesetround(FE_DOWNWARD);
+	if (read(fd, dummy, 1) < 0) {
+		printf("[FAIL]\taccess with downward rounding mode failed\n");
+		return 2;
+	}
+	if (fegetround() != FE_DOWNWARD) {
+		printf("[FAIL]\tusermode rounding mode clobbered\n");
+		return 3;
+	}
+
+	/* Note: the tests up to this point are quite safe and will only return
+	 * an error. But the exception mask setting can cause misbehaving kernel
+	 * to crash.
+	 */
+	feclearexcept(FE_ALL_EXCEPT);
+	feenableexcept(FE_ALL_EXCEPT);
+	if (read(fd, dummy, 1) < 0) {
+		printf("[FAIL]\taccess with fpu exceptions unmasked failed\n");
+		return 4;
+	}
+	if (fegetexcept() != FE_ALL_EXCEPT) {
+		printf("[FAIL]\tusermode fpu exception mask clobbered\n");
+		return 5;
+	}
+
+	printf("[OK]\ttest_fpu\n");
+	return 0;
+}