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: 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>

diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
index babea97b31a4307a90de73b86af7ffd93f4b1fbc..eaad5b20854ccf095a4447245554f8ecd48e0506 100644 (file)
--- a/tools/testing/selftests/kvm/dirty_log_test.c
+++ b/tools/testing/selftests/kvm/dirty_log_test.c
@@ -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"), "