KVM: selftests: Add a read-only mprotect() phase to mmu_stress_test

Add a third phase of mmu_stress_test to verify that mprotect()ing guest
memory to make it read-only doesn't cause explosions, e.g. to verify KVM
correctly handles the resulting mmu_notifier invalidations.

Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

tools/testing/selftests/kvm/mmu_stress_test.c

@@ -28,6 +28,10 @@ static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
 		GUEST_SYNC(i);
 	}
 
+	for (gpa = start_gpa; gpa < end_gpa; gpa += stride)
+		*((volatile uint64_t *)gpa);
+	GUEST_SYNC(2);
+
 	GUEST_ASSERT(0);
 }
 
@@ -95,6 +99,10 @@ static void *vcpu_worker(void *data)
 	run_vcpu(vcpu, 1);
 	rendezvous_with_boss();
 
+	/* Stage 2, read all of guest memory, which is now read-only. */
+	run_vcpu(vcpu, 2);
+	rendezvous_with_boss();
+
 	return NULL;
 }
 
@@ -175,7 +183,7 @@ int main(int argc, char *argv[])
 	const uint64_t start_gpa = SZ_4G;
 	const int first_slot = 1;
 
-	struct timespec time_start, time_run1, time_reset, time_run2;
+	struct timespec time_start, time_run1, time_reset, time_run2, time_ro;
 	uint64_t max_gpa, gpa, slot_size, max_mem, i;
 	int max_slots, slot, opt, fd;
 	bool hugepages = false;
@@ -279,14 +287,20 @@ int main(int argc, char *argv[])
 	rendezvous_with_vcpus(&time_reset, "reset");
 	rendezvous_with_vcpus(&time_run2, "run 2");
 
+	mprotect(mem, slot_size, PROT_READ);
+	rendezvous_with_vcpus(&time_ro, "mprotect RO");
+
+	time_ro    = timespec_sub(time_ro,     time_run2);
 	time_run2  = timespec_sub(time_run2,   time_reset);
-	time_reset = timespec_sub(time_reset, time_run1);
+	time_reset = timespec_sub(time_reset,  time_run1);
 	time_run1  = timespec_sub(time_run1,   time_start);
 
-	pr_info("run1 = %ld.%.9lds, reset = %ld.%.9lds, run2 =  %ld.%.9lds\n",
+	pr_info("run1 = %ld.%.9lds, reset = %ld.%.9lds, run2 = %ld.%.9lds, "
+		"ro = %ld.%.9lds\n",
 		time_run1.tv_sec, time_run1.tv_nsec,
 		time_reset.tv_sec, time_reset.tv_nsec,
-		time_run2.tv_sec, time_run2.tv_nsec);
+		time_run2.tv_sec, time_run2.tv_nsec,
+		time_ro.tv_sec, time_ro.tv_nsec);
 
 	/*
 	 * Delete even numbered slots (arbitrary) and unmap the first half of