KVM: selftests: Add configurable demand paging delay

When running the demand paging test with the -u option, the User Fault
FD handler essentially adds an arbitrary delay to page fault resolution.
To enable better simulation of a real demand paging scenario, add a
configurable delay to the UFFD handler.
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/demand_paging_test.c

@@ -145,6 +145,7 @@ bool quit_uffd_thread;
 struct uffd_handler_args {
 	int uffd;
 	int pipefd;
+	useconds_t delay;
 };
 
 static void *uffd_handler_thread_fn(void *arg)
@@ -152,6 +153,7 @@ static void *uffd_handler_thread_fn(void *arg)
 	struct uffd_handler_args *uffd_args = (struct uffd_handler_args *)arg;
 	int uffd = uffd_args->uffd;
 	int pipefd = uffd_args->pipefd;
+	useconds_t delay = uffd_args->delay;
 	int64_t pages = 0;
 
 	while (!quit_uffd_thread) {
@@ -212,6 +214,8 @@ static void *uffd_handler_thread_fn(void *arg)
 		if (!(msg.event & UFFD_EVENT_PAGEFAULT))
 			continue;
 
+		if (delay)
+			usleep(delay);
 		addr =  msg.arg.pagefault.address;
 		r = handle_uffd_page_request(uffd, addr);
 		if (r < 0)
@@ -223,7 +227,8 @@ static void *uffd_handler_thread_fn(void *arg)
 }
 
 static int setup_demand_paging(struct kvm_vm *vm,
-			       pthread_t *uffd_handler_thread, int pipefd)
+			       pthread_t *uffd_handler_thread, int pipefd,
+			       useconds_t uffd_delay)
 {
 	int uffd;
 	struct uffdio_api uffdio_api;
@@ -264,6 +269,7 @@ static int setup_demand_paging(struct kvm_vm *vm,
 
 	uffd_args.uffd = uffd;
 	uffd_args.pipefd = pipefd;
+	uffd_args.delay = uffd_delay;
 	pthread_create(uffd_handler_thread, NULL, uffd_handler_thread_fn,
 		       &uffd_args);
 
@@ -273,7 +279,8 @@ static int setup_demand_paging(struct kvm_vm *vm,
 #define GUEST_MEM_SHIFT 30 /* 1G */
 #define PAGE_SHIFT_4K  12
 
-static void run_test(enum vm_guest_mode mode, bool use_uffd)
+static void run_test(enum vm_guest_mode mode, bool use_uffd,
+		     useconds_t uffd_delay)
 {
 	pthread_t vcpu_thread;
 	pthread_t uffd_handler_thread;
@@ -341,7 +348,8 @@ static void run_test(enum vm_guest_mode mode, bool use_uffd)
 		r = pipe2(pipefd, O_CLOEXEC | O_NONBLOCK);
 		TEST_ASSERT(!r, "Failed to set up pipefd");
 
-		r = setup_demand_paging(vm, &uffd_handler_thread, pipefd[0]);
+		r = setup_demand_paging(vm, &uffd_handler_thread, pipefd[0],
+					uffd_delay);
 		if (r < 0)
 			exit(-r);
 	}
@@ -395,7 +403,7 @@ static void help(char *name)
 	int i;
 
 	puts("");
-	printf("usage: %s [-h] [-m mode] [-u]\n", name);
+	printf("usage: %s [-h] [-m mode] [-u] [-d uffd_delay_usec]\n", name);
 	printf(" -m: specify the guest mode ID to test\n"
 	       "     (default: test all supported modes)\n"
 	       "     This option may be used multiple times.\n"
@@ -404,7 +412,11 @@ static void help(char *name)
 		printf("         %d:    %s%s\n", i, vm_guest_mode_string(i),
 		       guest_modes[i].supported ? " (supported)" : "");
 	}
-	printf(" -u: Use User Fault FD to handle vCPU page faults.\n");
+	printf(" -u: use User Fault FD to handle vCPU page\n"
+	       "     faults.\n");
+	printf(" -d: add a delay in usec to the User Fault\n"
+	       "     FD handler to simulate demand paging\n"
+	       "     overheads. Ignored without -u.\n");
 	puts("");
 	exit(0);
 }
@@ -415,6 +427,7 @@ int main(int argc, char *argv[])
 	unsigned int mode;
 	int opt, i;
 	bool use_uffd = false;
+	useconds_t uffd_delay = 0;
 
 #ifdef __x86_64__
 	guest_mode_init(VM_MODE_PXXV48_4K, true, true);
@@ -437,7 +450,7 @@ int main(int argc, char *argv[])
 	guest_mode_init(VM_MODE_P40V48_4K, true, true);
 #endif
 
-	while ((opt = getopt(argc, argv, "hm:u")) != -1) {
+	while ((opt = getopt(argc, argv, "hm:ud:")) != -1) {
 		switch (opt) {
 		case 'm':
 			if (!mode_selected) {
@@ -453,6 +466,11 @@ int main(int argc, char *argv[])
 		case 'u':
 			use_uffd = true;
 			break;
+		case 'd':
+			uffd_delay = strtoul(optarg, NULL, 0);
+			TEST_ASSERT(uffd_delay >= 0,
+				    "A negative UFFD delay is not supported.");
+			break;
 		case 'h':
 		default:
 			help(argv[0]);
@@ -466,7 +484,7 @@ int main(int argc, char *argv[])
 		TEST_ASSERT(guest_modes[i].supported,
 			    "Guest mode ID %d (%s) not supported.",
 			    i, vm_guest_mode_string(i));
-		run_test(i, use_uffd);
+		run_test(i, use_uffd, uffd_delay);
 	}
 
 	return 0;