[PATCH 6.1 015/197] KVM: selftests: Fix a semaphore imbalance in the dirty ring logging test

Greg Kroah-Hartman
2024-02-20T20:58:36Z

From: Sean Christopherson <seanjc@google.com>

[ Upstream commit ba58f873cdeec30b6da48e28dd5782c5a3e1371b ]

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: Shaoqin Huang <shahuang@redhat.com>
Fixes: 84292e565951 ("KVM: selftests: Add dirty ring buffer test")
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Shaoqin Huang <shahuang@redhat.com>
Link: https://lore.kernel.org/r/20240202231831.354848-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/testing/selftests/kvm/dirty_log_test.c | 50 +++++++++++---------
1 file changed, 27 insertions(+), 23 deletions(-)

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index 8758c10ec850..ec40a33c29fd 100644
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -355,7 +355,10 @@ 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);

@@ -394,12 +397,6 @@ 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 */
@@ -412,7 +409,6 @@ 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",
@@ -431,7 +427,6 @@ 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,
},
};
@@ -492,14 +487,6 @@ 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;
@@ -701,6 +688,7 @@ 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",
@@ -772,12 +760,22 @@ 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) {
@@ -803,15 +801,21 @@ 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"), "
--
2.43.0