KVM: selftests: Extend MSRs test to validate vCPUs without supported features

Add a third vCPUs to the MSRs test that runs with all features disabled in
the vCPU's CPUID model, to verify that KVM does the right thing with
respect to emulating accesses to MSRs that shouldn't exist.  Use the same
VM to verify that KVM is honoring the vCPU model, e.g. isn't looking at
per-VM state when emulating MSR accesses.

Link: https://lore.kernel.org/r/20250919223258.1604852-48-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/x86/msrs_test.c

@@ -303,12 +303,17 @@ static void test_msrs(void)
 		MSR_TEST(MSR_IA32_PL3_SSP, canonical_val, canonical_val | 1, SHSTK),
 	};
 
+	const struct kvm_x86_cpu_feature feat_none = X86_FEATURE_NONE;
+	const struct kvm_x86_cpu_feature feat_lm = X86_FEATURE_LM;
+
 	/*
-	 * Create two vCPUs, but run them on the same task, to validate KVM's
+	 * Create three vCPUs, but run them on the same task, to validate KVM's
 	 * context switching of MSR state.  Don't pin the task to a pCPU to
-	 * also validate KVM's handling of cross-pCPU migration.
+	 * also validate KVM's handling of cross-pCPU migration.  Use the full
+	 * set of features for the first two vCPUs, but clear all features in
+	 * third vCPU in order to test both positive and negative paths.
 	 */
-	const int NR_VCPUS = 2;
+	const int NR_VCPUS = 3;
 	struct kvm_vcpu *vcpus[NR_VCPUS];
 	struct kvm_vm *vm;
 
@@ -323,6 +328,23 @@ static void test_msrs(void)
 	sync_global_to_guest(vm, msrs);
 	sync_global_to_guest(vm, ignore_unsupported_msrs);
 
+	/*
+	 * Clear features in the "unsupported features" vCPU.  This needs to be
+	 * done before the first vCPU run as KVM's ABI is that guest CPUID is
+	 * immutable once the vCPU has been run.
+	 */
+	for (idx = 0; idx < ARRAY_SIZE(__msrs); idx++) {
+		/*
+		 * Don't clear LM; selftests are 64-bit only, and KVM doesn't
+		 * honor LM=0 for MSRs that are supposed to exist if and only
+		 * if the vCPU is a 64-bit model.  Ditto for NONE; clearing a
+		 * fake feature flag will result in false failures.
+		 */
+		if (memcmp(&msrs[idx].feature, &feat_lm, sizeof(feat_lm)) &&
+		    memcmp(&msrs[idx].feature, &feat_none, sizeof(feat_none)))
+			vcpu_clear_cpuid_feature(vcpus[2], msrs[idx].feature);
+	}
+
 	for (idx = 0; idx < ARRAY_SIZE(__msrs); idx++) {
 		sync_global_to_guest(vm, idx);