KVM: x86: do not attempt TSC synchronization on guest writes

KVM special-cases writes to MSR_IA32_TSC so that all CPUs have
the same base for the TSC.  This logic is complicated, and we
do not want it to have any effect once the VM is started.

In particular, if any guest started to synchronize its TSCs
with writes to MSR_IA32_TSC rather than MSR_IA32_TSC_ADJUST,
the additional effect of kvm_write_tsc code would be uncharted
territory.

Therefore, this patch makes writes to MSR_IA32_TSC behave
essentially the same as writes to MSR_IA32_TSC_ADJUST when
they come from the guest.  A new selftest (which passes
both before and after the patch) checks the current semantics
of writes to MSR_IA32_TSC and MSR_IA32_TSC_ADJUST originating
from both the host and the guest.

Upcoming work to remove the special side effects
of host-initiated writes to MSR_IA32_TSC and MSR_IA32_TSC_ADJUST
will be able to build onto this test, adjusting the host side
to use the new APIs and achieve the same effect.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 168 insertions, 0 deletions

diff --git a/tools/testing/selftests/kvm/x86_64/tsc_msrs_test.c b/tools/testing/selftests/kvm/x86_64/tsc_msrs_test.c
new file mode 100644
index 000000000000..f8e761149daa
--- /dev/null
+++ b/tools/testing/selftests/kvm/x86_64/tsc_msrs_test.c
@@ -0,0 +1,168 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Tests for MSR_IA32_TSC and MSR_IA32_TSC_ADJUST.
+ *
+ * Copyright (C) 2020, Red Hat, Inc.
+ */
+#include <stdio.h>
+#include <string.h>
+#include "kvm_util.h"
+#include "processor.h"
+
+#define VCPU_ID 0
+
+#define UNITY                  (1ull << 30)
+#define HOST_ADJUST            (UNITY * 64)
+#define GUEST_STEP             (UNITY * 4)
+#define ROUND(x)               ((x + UNITY / 2) & -UNITY)
+#define rounded_rdmsr(x)       ROUND(rdmsr(x))
+#define rounded_host_rdmsr(x)  ROUND(vcpu_get_msr(vm, 0, x))
+
+#define GUEST_ASSERT_EQ(a, b) do {				\
+	__typeof(a) _a = (a);					\
+	__typeof(b) _b = (b);					\
+	if (_a != _b)						\
+                ucall(UCALL_ABORT, 4,				\
+                        "Failed guest assert: "			\
+                        #a " == " #b, __LINE__, _a, _b);	\
+  } while(0)
+
+static void guest_code(void)
+{
+	u64 val = 0;
+
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC), val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/* Guest: writes to MSR_IA32_TSC affect both MSRs.  */
+	val = 1ull * GUEST_STEP;
+	wrmsr(MSR_IA32_TSC, val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC), val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/* Guest: writes to MSR_IA32_TSC_ADJUST affect both MSRs.  */
+	GUEST_SYNC(2);
+	val = 2ull * GUEST_STEP;
+	wrmsr(MSR_IA32_TSC_ADJUST, val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC), val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/* Host: setting the TSC offset.  */
+	GUEST_SYNC(3);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC), HOST_ADJUST + val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/*
+	 * Guest: writes to MSR_IA32_TSC_ADJUST do not destroy the
+	 * host-side offset and affect both MSRs.
+	 */
+	GUEST_SYNC(4);
+	val = 3ull * GUEST_STEP;
+	wrmsr(MSR_IA32_TSC_ADJUST, val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC), HOST_ADJUST + val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/*
+	 * Guest: writes to MSR_IA32_TSC affect both MSRs, so the host-side
+	 * offset is now visible in MSR_IA32_TSC_ADJUST.
+	 */
+	GUEST_SYNC(5);
+	val = 4ull * GUEST_STEP;
+	wrmsr(MSR_IA32_TSC, val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC), val);
+	GUEST_ASSERT_EQ(rounded_rdmsr(MSR_IA32_TSC_ADJUST), val - HOST_ADJUST);
+
+	GUEST_DONE();
+}
+
+static void run_vcpu(struct kvm_vm *vm, uint32_t vcpuid, int stage)
+{
+	struct ucall uc;
+
+	vcpu_args_set(vm, vcpuid, 1, vcpuid);
+
+	vcpu_ioctl(vm, vcpuid, KVM_RUN, NULL);
+
+	switch (get_ucall(vm, vcpuid, &uc)) {
+	case UCALL_SYNC:
+		TEST_ASSERT(!strcmp((const char *)uc.args[0], "hello") &&
+                            uc.args[1] == stage + 1, "Stage %d: Unexpected register values vmexit, got %lx",
+                            stage + 1, (ulong)uc.args[1]);
+		return;
+	case UCALL_DONE:
+		return;
+	case UCALL_ABORT:
+		TEST_ASSERT(false, "%s at %s:%ld\n" \
+			    "\tvalues: %#lx, %#lx", (const char *)uc.args[0],
+			    __FILE__, uc.args[1], uc.args[2], uc.args[3]);
+	default:
+		TEST_ASSERT(false, "Unexpected exit: %s",
+			    exit_reason_str(vcpu_state(vm, vcpuid)->exit_reason));
+	}
+}
+
+int main(void)
+{
+	struct kvm_vm *vm;
+	uint64_t val;
+
+	vm = vm_create_default(VCPU_ID, 0, guest_code);
+	vcpu_set_cpuid(vm, VCPU_ID, kvm_get_supported_cpuid());
+
+	val = 0;
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC), val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/* Guest: writes to MSR_IA32_TSC affect both MSRs.  */
+	run_vcpu(vm, VCPU_ID, 1);
+	val = 1ull * GUEST_STEP;
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC), val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/* Guest: writes to MSR_IA32_TSC_ADJUST affect both MSRs.  */
+	run_vcpu(vm, VCPU_ID, 2);
+	val = 2ull * GUEST_STEP;
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC), val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/*
+	 * Host: writes to MSR_IA32_TSC set the host-side offset
+	 * and therefore do not change MSR_IA32_TSC_ADJUST.
+	 */
+	vcpu_set_msr(vm, 0, MSR_IA32_TSC, HOST_ADJUST + val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC), HOST_ADJUST + val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC_ADJUST), val);
+	run_vcpu(vm, VCPU_ID, 3);
+
+	/* Host: writes to MSR_IA32_TSC_ADJUST do not modify the TSC.  */
+	vcpu_set_msr(vm, 0, MSR_IA32_TSC_ADJUST, UNITY * 123456);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC), HOST_ADJUST + val);
+	ASSERT_EQ(vcpu_get_msr(vm, 0, MSR_IA32_TSC_ADJUST), UNITY * 123456);
+
+	/* Restore previous value.  */
+	vcpu_set_msr(vm, 0, MSR_IA32_TSC_ADJUST, val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC), HOST_ADJUST + val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/*
+	 * Guest: writes to MSR_IA32_TSC_ADJUST do not destroy the
+	 * host-side offset and affect both MSRs.
+	 */
+	run_vcpu(vm, VCPU_ID, 4);
+	val = 3ull * GUEST_STEP;
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC), HOST_ADJUST + val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC_ADJUST), val);
+
+	/*
+	 * Guest: writes to MSR_IA32_TSC affect both MSRs, so the host-side
+	 * offset is now visible in MSR_IA32_TSC_ADJUST.
+	 */
+	run_vcpu(vm, VCPU_ID, 5);
+	val = 4ull * GUEST_STEP;
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC), val);
+	ASSERT_EQ(rounded_host_rdmsr(MSR_IA32_TSC_ADJUST), val - HOST_ADJUST);
+
+	kvm_vm_free(vm);
+
+	return 0;
+}