KVM: arm64: selftests: Add test for SVE host corruption

Until recently, the kernel could unexpectedly discard SVE state for a
period after a KVM_RUN ioctl, when the guest did not execute any
FPSIMD/SVE/SME instructions. We fixed that issue in commit:

fbc7e61195e2 ("KVM: arm64: Unconditionally save+flush host FPSIMD/SVE/SME state")

Add a test which tries to provoke that issue by manipulating SVE state
before/after running a guest which does not execute any FPSIMD/SVE/SME
instructions. The test executes a handful of iterations to miminize
the risk that the issue is masked by preemption.

Signed-off--by: Mark Rutland <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>
Acked-by: Mark Rutland <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[maz: Restored MR's SoB, fixed commit message according to MR's write-up]
Signed-off-by: Marc Zyngier <[e-mail]>

tools/testing/selftests/kvm/Makefile.kvm

···
 TEST_GEN_PROGS_arm64 += arm64/aarch32_id_regs
 TEST_GEN_PROGS_arm64 += arm64/arch_timer_edge_cases
 TEST_GEN_PROGS_arm64 += arm64/debug-exceptions
+TEST_GEN_PROGS_arm64 += arm64/host_sve
 TEST_GEN_PROGS_arm64 += arm64/hypercalls
 TEST_GEN_PROGS_arm64 += arm64/mmio_abort
 TEST_GEN_PROGS_arm64 += arm64/page_fault_test

tools/testing/selftests/kvm/arm64/host_sve.c

···
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ * Host SVE: Check FPSIMD/SVE/SME save/restore over KVM_RUN ioctls.
+ *
+ * Copyright 2025 Arm, Ltd
+ */
+
+#include <errno.h>
+#include <signal.h>
+#include <sys/auxv.h>
+#include <asm/kvm.h>
+#include <kvm_util.h>
+
+#include "ucall_common.h"
+
+static void guest_code(void)
+{
+	for (int i = 0; i < 10; i++) {
+		GUEST_UCALL_NONE();
+	}
+
+	GUEST_DONE();
+}
+
+void handle_sigill(int sig, siginfo_t *info, void *ctx)
+{
+	ucontext_t *uctx = ctx;
+
+	printf("  < host signal %d >\n", sig);
+
+	/*
+	 * Skip the UDF
+	 */
+	uctx->uc_mcontext.pc += 4;
+}
+
+void register_sigill_handler(void)
+{
+	struct sigaction sa = {
+		.sa_sigaction = handle_sigill,
+		.sa_flags = SA_SIGINFO,
+	};
+	sigaction(SIGILL, &sa, NULL);
+}
+
+static void do_sve_roundtrip(void)
+{
+	unsigned long before, after;
+
+	/*
+	 * Set all bits in a predicate register, force a save/restore via a
+	 * SIGILL (which handle_sigill() will recover from), then report
+	 * whether the value has changed.
+	 */
+	asm volatile(
+	"	.arch_extension sve\n"
+	"	ptrue	p0.B\n"
+	"	cntp	%[before], p0, p0.B\n"
+	"	udf #0\n"
+	"	cntp	%[after], p0, p0.B\n"
+	: [before] "=r" (before),
+	  [after] "=r" (after)
+	:
+	: "p0"
+	);
+
+	if (before != after) {
+		TEST_FAIL("Signal roundtrip discarded predicate bits (%ld => %ld)\n",
+			  before, after);
+	} else {
+		printf("Signal roundtrip preserved predicate bits (%ld => %ld)\n",
+		       before, after);
+	}
+}
+
+static void test_run(void)
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	struct ucall uc;
+	bool guest_done = false;
+
+	register_sigill_handler();
+
+	vm = vm_create_with_one_vcpu(&vcpu, guest_code);
+
+	do_sve_roundtrip();
+
+	while (!guest_done) {
+
+		printf("Running VCPU...\n");
+		vcpu_run(vcpu);
+
+		switch (get_ucall(vcpu, &uc)) {
+		case UCALL_NONE:
+			do_sve_roundtrip();
+			do_sve_roundtrip();
+			break;
+		case UCALL_DONE:
+			guest_done = true;
+			break;
+		case UCALL_ABORT:
+			REPORT_GUEST_ASSERT(uc);
+			break;
+		default:
+			TEST_FAIL("Unexpected guest exit");
+		}
+	}
+
+	kvm_vm_free(vm);
+}
+
+int main(void)
+{
+	/*
+	 * This is testing the host environment, we don't care about
+	 * guest SVE support.
+	 */
+	if (!(getauxval(AT_HWCAP) & HWCAP_SVE)) {
+		printf("SVE not supported\n");
+		return KSFT_SKIP;
+	}
+
+	test_run();
+	return 0;
+}