KVM: selftests: Add wrfract to common guest code

Wrfract will be used by the dirty logging perf test introduced later in
this series to dirty memory sparsely.

This series was tested by running the following invocations on an Intel
Skylake machine:
dirty_log_perf_test -b 20m -i 100 -v 64
dirty_log_perf_test -b 20g -i 5 -v 4
dirty_log_perf_test -b 4g -i 5 -v 32
demand_paging_test -b 20m -v 64
demand_paging_test -b 20g -v 4
demand_paging_test -b 4g -v 32
All behaved as expected.
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20201027233733.1484855-5-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/demand_paging_test.c

@@ -266,6 +266,8 @@ static void run_test(enum vm_guest_mode mode, bool use_uffd,
 
 	vm = create_vm(mode, vcpus, vcpu_memory_bytes);
 
+	perf_test_args.wr_fract = 1;
+
 	guest_data_prototype = malloc(perf_test_args.host_page_size);
 	TEST_ASSERT(guest_data_prototype,
 		    "Failed to allocate buffer for guest data pattern");

tools/testing/selftests/kvm/include/perf_test_util.h

@@ -46,6 +46,7 @@ struct perf_test_args {
 	struct kvm_vm *vm;
 	uint64_t host_page_size;
 	uint64_t guest_page_size;
+	int wr_fract;
 
 	struct vcpu_args vcpu_args[MAX_VCPUS];
 };
@@ -72,7 +73,10 @@ static void guest_code(uint32_t vcpu_id)
 	for (i = 0; i < pages; i++) {
 		uint64_t addr = gva + (i * perf_test_args.guest_page_size);
 
-		*(uint64_t *)addr = 0x0123456789ABCDEF;
+		if (i % perf_test_args.wr_fract == 0)
+			*(uint64_t *)addr = 0x0123456789ABCDEF;
+		else
+			READ_ONCE(*(uint64_t *)addr);
 	}
 
 	GUEST_SYNC(1);