KVM: selftests: Use KVM's reported TSC KHz as L0's frequency (sanity checked)

Use KVM's reported default TSC frequency as L0's frequency instead of
calibrating L0's frequency using RDTSC.  Trying to calibrate the frequency
in userspace is all but guaranteed to be wildly inaccurate, e.g. due to
interrupts, preemption, etc, and no known CPU supports TSC scaling without
also having a constant TSC.  I.e. there's no need to calibrate the current
frequency, as it should also be the same as KVM's default frequency.

Opportunistically sanity check KVM's reported frequency against what is
observed via RDTSC, e.g. so that KVM_GET_TSC_KHZ or platform issues show up
much earlier in the test, before VMs get involved.

Link: https://patch.msgid.link/20260930173635.3362655-14-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 17 insertions, 8 deletions

diff --git a/tools/testing/selftests/kvm/x86/nested_tsc_scaling_test.c b/tools/testing/selftests/kvm/x86/nested_tsc_scaling_test.c
index d790ab36a198..a78585af0ffd 100644
--- a/tools/testing/selftests/kvm/x86/nested_tsc_scaling_test.c
+++ b/tools/testing/selftests/kvm/x86/nested_tsc_scaling_test.c
@@ -137,7 +137,7 @@ static void l1_guest_code(void *data)
 
 static void test_tsc_scaling(u64 l0_tsc_freq, u64 l1_scale_factor, u64 l2_scale_factor)
 {
-	u64 tsc_khz, l1_tsc_freq, l2_tsc_freq;
+	u64 l1_tsc_freq, l2_tsc_freq;
 	struct kvm_vcpu *vcpu;
 	struct kvm_vm *vm;
 	gva_t guest_gva;
@@ -159,11 +159,8 @@ static void test_tsc_scaling(u64 l0_tsc_freq, u64 l1_scale_factor, u64 l2_scale_
 
 	vcpu_args_set(vcpu, 1, guest_gva);
 
-	tsc_khz = __vcpu_ioctl(vcpu, KVM_GET_TSC_KHZ, NULL);
-	TEST_ASSERT(tsc_khz != -1, "vcpu ioctl KVM_GET_TSC_KHZ failed");
-
 	/* scale down L1's TSC frequency */
-	vcpu_ioctl(vcpu, KVM_SET_TSC_KHZ, (void *) (tsc_khz / l1_scale_factor));
+	vcpu_ioctl(vcpu, KVM_SET_TSC_KHZ, (void *) (l0_tsc_freq / l1_scale_factor));
 
 	/* L1 will communicate its frequency before the L2 check.*/
 	l1_tsc_freq = 0;
@@ -212,18 +209,30 @@ done:
 int main(int argc, char *argv[])
 {
 	u64 l0_tsc_freq, tsc_start, tsc_end, l1_scale, l2_scale;
+	struct kvm_vm *vm;
 
 	TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_VMX) ||
 		     kvm_cpu_has(X86_FEATURE_SVM));
 	TEST_REQUIRE(kvm_has_cap(KVM_CAP_TSC_CONTROL));
 	TEST_REQUIRE(sys_clocksource_is_based_on_tsc());
 
+	/*
+	 * Create a dummy VM to get KVM's default TSC frequency.  All CPUs that
+	 * support TSC scaling should have a constant TSC, i.e. there's no need
+	 * to calibrate the "real" TSC.  But do sanity check that the observed
+	 * TSC is within range of KVM's reported TSC frequency.
+	 */
+	vm = vm_create_barebones();
+	l0_tsc_freq = (u64)__vm_ioctl(vm, KVM_GET_TSC_KHZ, NULL) * 1000;
+	TEST_ASSERT(l0_tsc_freq, "vcpu ioctl KVM_GET_TSC_KHZ failed");
+	kvm_vm_free(vm);
+
+	printf("L0 TSC frequency is: %lu\n", l0_tsc_freq);
+
 	tsc_start = rdtsc();
 	sleep(1);
 	tsc_end = rdtsc();
-
-	l0_tsc_freq = tsc_end - tsc_start;
-	printf("real TSC frequency is around: %lu\n", l0_tsc_freq);
+	host_check_tsc_freq(0, tsc_end - tsc_start, l0_tsc_freq);
 
 	/* Scale L1 "down" and L2 "up" at a random factor from 2 to 10. */
 	l1_scale = (kvm_random_u32(&kvm_rng) % 9) + 2;