KVM: selftests: Reload "good" vCPU state if vCPU hits shutdown

Reload known good vCPU state if the vCPU triple faults in any of the
race_sync_regs() subtests, e.g. if KVM successfully injects an exception
(the vCPU isn't configured to handle exceptions).  On Intel, the VMCS
is preserved even after shutdown, but AMD's APM states that the VMCB is
undefined after a shutdown and so KVM synthesizes an INIT to sanitize
vCPU/VMCB state, e.g. to guard against running with a garbage VMCB.

The synthetic INIT results in the vCPU never exiting to userspace, as it
gets put into Real Mode at the reset vector, which is full of zeros (as is
GPA 0 and beyond), and so executes ADD for a very, very long time.

Fixes: 60c4063b ("KVM: selftests: Extend x86's sync_regs_test to check for event vector races")
Cc: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230817233430.1416463-2-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/x86_64/sync_regs_test.c

@@ -152,6 +152,7 @@ static noinline void *race_sregs_cr4(void *arg)
 static void race_sync_regs(void *racer)
 {
 	const time_t TIMEOUT = 2; /* seconds, roughly */
+	struct kvm_x86_state *state;
 	struct kvm_translation tr;
 	struct kvm_vcpu *vcpu;
 	struct kvm_run *run;
@@ -166,6 +167,9 @@ static void race_sync_regs(void *racer)
 	vcpu_run(vcpu);
 	run->kvm_valid_regs = 0;
 
+	/* Save state *before* spawning the thread that mucks with vCPU state. */
+	state = vcpu_save_state(vcpu);
+
 	/*
 	 * Selftests run 64-bit guests by default, both EFER.LME and CR4.PAE
 	 * should already be set in guest state.
@@ -179,7 +183,14 @@ static void race_sync_regs(void *racer)
 	TEST_ASSERT_EQ(pthread_create(&thread, NULL, racer, (void *)run), 0);
 
 	for (t = time(NULL) + TIMEOUT; time(NULL) < t;) {
-		__vcpu_run(vcpu);
+		/*
+		 * Reload known good state if the vCPU triple faults, e.g. due
+		 * to the unhandled #GPs being injected.  VMX preserves state
+		 * on shutdown, but SVM synthesizes an INIT as the VMCB state
+		 * is architecturally undefined on triple fault.
+		 */
+		if (!__vcpu_run(vcpu) && run->exit_reason == KVM_EXIT_SHUTDOWN)
+			vcpu_load_state(vcpu, state);
 
 		if (racer == race_sregs_cr4) {
 			tr = (struct kvm_translation) { .linear_address = 0 };
@@ -190,6 +201,7 @@ static void race_sync_regs(void *racer)
 	TEST_ASSERT_EQ(pthread_cancel(thread), 0);
 	TEST_ASSERT_EQ(pthread_join(thread, NULL), 0);
 
+	kvm_x86_state_cleanup(state);
 	kvm_vm_free(vm);
 }