[PATCH 2/2] KVM: selftests: Test forced instruction emulation in dirty log test (x86 only)

[Sean Christopherson <seanjc@google.com>]

Add forced emulation of MOV and LOCK CMPXCHG instructions in the dirty log
test's guest code to verify that KVM's emulator marks pages dirty as
expected (and obviously to verify the emulator works at all).  In the long
term, the guest code would ideally hammer more of KVM's emulator, but for
starters, cover the two major paths: writes and atomics.

To minimize #ifdeffery, wrap only the related code that is x86 specific,
unnecessariliy synchronizing an extra boolean to the guest is far from the
end of the world.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 tools/testing/selftests/kvm/dirty_log_test.c | 36 ++++++++++++++++++--
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index eaad5b20854c..ff1d1c7f05d8 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -92,6 +92,29 @@ static uint64_t guest_test_phys_mem;
  */
 static uint64_t guest_test_virt_mem = DEFAULT_GUEST_TEST_MEM;
 
+static bool is_forced_emulation_enabled;
+
+static void guest_write_memory(uint64_t *mem, uint64_t val, uint64_t rand)
+{
+#ifdef __x86_64__
+	if (is_forced_emulation_enabled && (rand & 1)) {
+		if (rand & 2) {
+			__asm__ __volatile__(KVM_FEP "movq %1, %0"
+					     : "+m" (*mem)
+					     : "r" (val) : "memory");
+		} else {
+			uint64_t __old = READ_ONCE(*mem);
+
+			__asm__ __volatile__(KVM_FEP LOCK_PREFIX "cmpxchgq %[new], %[ptr]"
+					     : [ptr] "+m" (*mem), [old] "+a" (__old)
+					     : [new]"r" (val) : "memory", "cc");
+		}
+	} else
+#endif
+
+	*mem = val;
+}
+
 /*
  * Continuously write to the first 8 bytes of a random pages within
  * the testing memory region.
@@ -114,11 +137,13 @@ static void guest_code(void)
 
 	while (true) {
 		for (i = 0; i < TEST_PAGES_PER_LOOP; i++) {
+			uint64_t rand = READ_ONCE(random_array[i]);
+
 			addr = guest_test_virt_mem;
-			addr += (READ_ONCE(random_array[i]) % guest_num_pages)
-				* guest_page_size;
+			addr += (rand % guest_num_pages) * guest_page_size;
 			addr = align_down(addr, host_page_size);
-			*(uint64_t *)addr = READ_ONCE(iteration);
+
+			guest_write_memory((void *)addr, READ_ONCE(iteration), rand);
 		}
 
 		/* Tell the host that we need more random numbers */
@@ -772,6 +797,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
 	sync_global_to_guest(vm, guest_page_size);
 	sync_global_to_guest(vm, guest_test_virt_mem);
 	sync_global_to_guest(vm, guest_num_pages);
+	sync_global_to_guest(vm, is_forced_emulation_enabled);
 
 	/* Start the iterations */
 	iteration = 1;
@@ -875,6 +901,10 @@ int main(int argc, char *argv[])
 	int opt, i;
 	sigset_t sigset;
 
+#ifdef __x86_64__
+	is_forced_emulation_enabled = kvm_is_forced_emulation_enabled();
+#endif
+
 	sem_init(&sem_vcpu_stop, 0, 0);
 	sem_init(&sem_vcpu_cont, 0, 0);
 
-- 
2.43.0.687.g38aa6559b0-goog