KVM: selftests: Precisely limit the number of guest loops in mmu_stress_test

Run the exact number of guest loops required in mmu_stress_test instead
of looping indefinitely in anticipation of adding more stages that run
different code (e.g. reads instead of writes).

Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/mmu_stress_test.c

@@ -20,12 +20,15 @@
 static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
 {
 	uint64_t gpa;
+	int i;
 
-	for (;;) {
+	for (i = 0; i < 2; i++) {
 		for (gpa = start_gpa; gpa < end_gpa; gpa += stride)
 			vcpu_arch_put_guest(*((volatile uint64_t *)gpa), gpa);
-		GUEST_SYNC(0);
+		GUEST_SYNC(i);
 	}
+
+	GUEST_ASSERT(0);
 }
 
 struct vcpu_info {
@@ -52,10 +55,18 @@ static void rendezvous_with_boss(void)
 	}
 }
 
-static void run_vcpu(struct kvm_vcpu *vcpu)
+static void assert_sync_stage(struct kvm_vcpu *vcpu, int stage)
+{
+	struct ucall uc;
+
+	TEST_ASSERT_EQ(get_ucall(vcpu, &uc), UCALL_SYNC);
+	TEST_ASSERT_EQ(uc.args[1], stage);
+}
+
+static void run_vcpu(struct kvm_vcpu *vcpu, int stage)
 {
 	vcpu_run(vcpu);
-	TEST_ASSERT_EQ(get_ucall(vcpu, NULL), UCALL_SYNC);
+	assert_sync_stage(vcpu, stage);
 }
 
 static void *vcpu_worker(void *data)
@@ -69,7 +80,8 @@ static void *vcpu_worker(void *data)
 
 	rendezvous_with_boss();
 
-	run_vcpu(vcpu);
+	/* Stage 0, write all of guest memory. */
+	run_vcpu(vcpu, 0);
 	rendezvous_with_boss();
 #ifdef __x86_64__
 	vcpu_sregs_get(vcpu, &sregs);
@@ -79,7 +91,8 @@ static void *vcpu_worker(void *data)
 #endif
 	rendezvous_with_boss();
 
-	run_vcpu(vcpu);
+	/* Stage 1, re-write all of guest memory. */
+	run_vcpu(vcpu, 1);
 	rendezvous_with_boss();
 
 	return NULL;