KVM: selftests: Add a CPUID testcase for KVM_SET_CPUID2 with runtime updates

Add a CPUID testcase to verify that KVM allows KVM_SET_CPUID2 after (or in
conjunction with) runtime updates.  This is a regression test for the bug
introduced by commit 93da6af ("KVM: x86: Defer runtime updates of
dynamic CPUID bits until CPUID emulation"), where KVM would incorrectly
reject KVM_SET_CPUID due to a not handling a pending runtime update on the
current CPUID, resulting in a false mismatch between the "old" and "new"
CPUID entries.

Link: https://lore.kernel.org/all/20251128123202.68424a95@imammedo
Link: https://patch.msgid.link/20251202015049.1167490-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

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

@@ -155,13 +155,28 @@ struct kvm_cpuid2 *vcpu_alloc_cpuid(struct kvm_vm *vm, vm_vaddr_t *p_gva, struct
 static void set_cpuid_after_run(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *ent;
+	struct kvm_sregs sregs;
 	int rc;
 	u32 eax, ebx, x;
 
 	/* Setting unmodified CPUID is allowed */
 	rc = __vcpu_set_cpuid(vcpu);
 	TEST_ASSERT(!rc, "Setting unmodified CPUID after KVM_RUN failed: %d", rc);
 
+	/*
+	 * Toggle CR4 bits that affect dynamic CPUID feature flags to verify
+	 * setting unmodified CPUID succeeds with runtime CPUID updates.
+	 */
+	vcpu_sregs_get(vcpu, &sregs);
+	if (kvm_cpu_has(X86_FEATURE_XSAVE))
+		sregs.cr4 ^= X86_CR4_OSXSAVE;
+	if (kvm_cpu_has(X86_FEATURE_PKU))
+		sregs.cr4 ^= X86_CR4_PKE;
+	vcpu_sregs_set(vcpu, &sregs);
+
+	rc = __vcpu_set_cpuid(vcpu);
+	TEST_ASSERT(!rc, "Setting unmodified CPUID after KVM_RUN failed: %d", rc);
+
 	/* Changing CPU features is forbidden */
 	ent = vcpu_get_cpuid_entry(vcpu, 0x7);
 	ebx = ent->ebx;