Commit ba58f873 authored by Sean Christopherson's avatar Sean Christopherson
Browse files

KVM: selftests: Fix a semaphore imbalance in the dirty ring logging test 



When finishing the final iteration of dirty_log_test testcase, set
host_quit _before_ the final "continue" so that the vCPU worker doesn't
run an extra iteration, and delete the hack-a-fix of an extra "continue"
from the dirty ring testcase.  This fixes a bug where the extra post to
sem_vcpu_cont may not be consumed, which results in failures in subsequent
runs of the testcases.  The bug likely was missed during development as
x86 supports only a single "guest mode", i.e. there aren't any subsequent
testcases after the dirty ring test, because for_each_guest_mode() only
runs a single iteration.

For the regular dirty log testcases, letting the vCPU run one extra
iteration is a non-issue as the vCPU worker waits on sem_vcpu_cont if and
only if the worker is explicitly told to stop (vcpu_sync_stop_requested).
But for the dirty ring test, which needs to periodically stop the vCPU to
reap the dirty ring, letting the vCPU resume the guest _after_ the last
iteration means the vCPU will get stuck without an extra "continue".

However, blindly firing off an post to sem_vcpu_cont isn't guaranteed to
be consumed, e.g. if the vCPU worker sees host_quit==true before resuming
the guest.  This results in a dangling sem_vcpu_cont, which leads to
subsequent iterations getting out of sync, as the vCPU worker will
continue on before the main task is ready for it to resume the guest,
leading to a variety of asserts, e.g.

  ==== Test Assertion Failure ====
  dirty_log_test.c:384: dirty_ring_vcpu_ring_full
  pid=14854 tid=14854 errno=22 - Invalid argument
     1  0x00000000004033eb: dirty_ring_collect_dirty_pages at dirty_log_test.c:384
     2  0x0000000000402d27: log_mode_collect_dirty_pages at dirty_log_test.c:505
     3   (inlined by) run_test at dirty_log_test.c:802
     4  0x0000000000403dc7: for_each_guest_mode at guest_modes.c:100
     5  0x0000000000401dff: main at dirty_log_test.c:941 (discriminator 3)
     6  0x0000ffff9be173c7: ?? ??:0
     7  0x0000ffff9be1749f: ?? ??:0
     8  0x000000000040206f: _start at ??:?
  Didn't continue vcpu even without ring full

Alternatively, the test could simply reset the semaphores before each
testcase, but papering over hacks with more hacks usually ends in tears.

Reported-by: default avatarShaoqin Huang <shahuang@redhat.com>
Fixes: 84292e56 ("KVM: selftests: Add dirty ring buffer test")
Reviewed-by: default avatarPeter Xu <peterx@redhat.com>
Reviewed-by: default avatarShaoqin Huang <shahuang@redhat.com>
Link: https://lore.kernel.org/r/20240202231831.354848-1-seanjc@google.com


Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 9e62797f

tools/testing/selftests/kvm/dirty_log_test.c

+27 −23
Original line number Diff line number Diff line
@@ -376,7 +376,10 @@ static void dirty_ring_collect_dirty_pages(struct kvm_vcpu *vcpu, int slot, 


	cleared = kvm_vm_reset_dirty_ring(vcpu->vm);



	/* Cleared pages should be the same as collected */

	/*

	 * Cleared pages should be the same as collected, as KVM is supposed to

	 * clear only the entries that have been harvested.

	 */

	TEST_ASSERT(cleared == count, "Reset dirty pages (%u) mismatch "

		    "with collected (%u)", cleared, count);
@@ -415,12 +418,6 @@ static void dirty_ring_after_vcpu_run(struct kvm_vcpu *vcpu, int ret, int err) 
	}

}



static void dirty_ring_before_vcpu_join(void)

{

	/* Kick another round of vcpu just to make sure it will quit */

	sem_post(&sem_vcpu_cont);

}



struct log_mode {

	const char *name;

	/* Return true if this mode is supported, otherwise false */
@@ -433,7 +430,6 @@ struct log_mode { 
				     uint32_t *ring_buf_idx);

	/* Hook to call when after each vcpu run */

	void (*after_vcpu_run)(struct kvm_vcpu *vcpu, int ret, int err);

	void (*before_vcpu_join) (void);

} log_modes[LOG_MODE_NUM] = {

	{

		.name = "dirty-log",
@@ -452,7 +448,6 @@ struct log_mode { 
		.supported = dirty_ring_supported,

		.create_vm_done = dirty_ring_create_vm_done,

		.collect_dirty_pages = dirty_ring_collect_dirty_pages,

		.before_vcpu_join = dirty_ring_before_vcpu_join,

		.after_vcpu_run = dirty_ring_after_vcpu_run,

	},

};
@@ -513,14 +508,6 @@ static void log_mode_after_vcpu_run(struct kvm_vcpu *vcpu, int ret, int err) 
		mode->after_vcpu_run(vcpu, ret, err);

}



static void log_mode_before_vcpu_join(void)

{

	struct log_mode *mode = &log_modes[host_log_mode];



	if (mode->before_vcpu_join)

		mode->before_vcpu_join();

}



static void generate_random_array(uint64_t *guest_array, uint64_t size)

{

	uint64_t i;
@@ -719,6 +706,7 @@ static void run_test(enum vm_guest_mode mode, void *arg) 
	struct kvm_vm *vm;

	unsigned long *bmap;

	uint32_t ring_buf_idx = 0;

	int sem_val;



	if (!log_mode_supported()) {

		print_skip("Log mode '%s' not supported",
@@ -788,12 +776,22 @@ static void run_test(enum vm_guest_mode mode, void *arg) 
	/* Start the iterations */

	iteration = 1;

	sync_global_to_guest(vm, iteration);

	host_quit = false;

	WRITE_ONCE(host_quit, false);

	host_dirty_count = 0;

	host_clear_count = 0;

	host_track_next_count = 0;

	WRITE_ONCE(dirty_ring_vcpu_ring_full, false);



	/*

	 * Ensure the previous iteration didn't leave a dangling semaphore, i.e.

	 * that the main task and vCPU worker were synchronized and completed

	 * verification of all iterations.

	 */

	sem_getvalue(&sem_vcpu_stop, &sem_val);

	TEST_ASSERT_EQ(sem_val, 0);

	sem_getvalue(&sem_vcpu_cont, &sem_val);

	TEST_ASSERT_EQ(sem_val, 0);



	pthread_create(&vcpu_thread, NULL, vcpu_worker, vcpu);



	while (iteration < p->iterations) {
@@ -819,15 +817,21 @@ static void run_test(enum vm_guest_mode mode, void *arg) 
		assert(host_log_mode == LOG_MODE_DIRTY_RING ||

		       atomic_read(&vcpu_sync_stop_requested) == false);

		vm_dirty_log_verify(mode, bmap);

		sem_post(&sem_vcpu_cont);



		iteration++;

		/*

		 * Set host_quit before sem_vcpu_cont in the final iteration to

		 * ensure that the vCPU worker doesn't resume the guest.  As

		 * above, the dirty ring test may stop and wait even when not

		 * explicitly request to do so, i.e. would hang waiting for a

		 * "continue" if it's allowed to resume the guest.

		 */

		if (++iteration == p->iterations)

			WRITE_ONCE(host_quit, true);



		sem_post(&sem_vcpu_cont);

		sync_global_to_guest(vm, iteration);

	}



	/* Tell the vcpu thread to quit */

	host_quit = true;

	log_mode_before_vcpu_join();

	pthread_join(vcpu_thread, NULL);



	pr_info("Total bits checked: dirty (%"PRIu64"), clear (%"PRIu64"), "