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>
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@ -376,7 +376,10 @@ static void dirty_ring_collect_dirty_pages(struct kvm_vcpu *vcpu, int slot,
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cleared = kvm_vm_reset_dirty_ring(vcpu->vm);
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/* Cleared pages should be the same as collected */
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/*
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* Cleared pages should be the same as collected, as KVM is supposed to
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* clear only the entries that have been harvested.
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*/
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TEST_ASSERT(cleared == count, "Reset dirty pages (%u) mismatch "
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"with collected (%u)", cleared, count);
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@ -415,12 +418,6 @@ static void dirty_ring_after_vcpu_run(struct kvm_vcpu *vcpu, int ret, int err)
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}
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}
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static void dirty_ring_before_vcpu_join(void)
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{
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/* Kick another round of vcpu just to make sure it will quit */
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sem_post(&sem_vcpu_cont);
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}
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struct log_mode {
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const char *name;
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/* Return true if this mode is supported, otherwise false */
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@ -433,7 +430,6 @@ struct log_mode {
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uint32_t *ring_buf_idx);
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/* Hook to call when after each vcpu run */
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void (*after_vcpu_run)(struct kvm_vcpu *vcpu, int ret, int err);
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void (*before_vcpu_join) (void);
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} log_modes[LOG_MODE_NUM] = {
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{
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.name = "dirty-log",
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@ -452,7 +448,6 @@ struct log_mode {
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.supported = dirty_ring_supported,
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.create_vm_done = dirty_ring_create_vm_done,
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.collect_dirty_pages = dirty_ring_collect_dirty_pages,
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.before_vcpu_join = dirty_ring_before_vcpu_join,
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.after_vcpu_run = dirty_ring_after_vcpu_run,
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},
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};
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@ -513,14 +508,6 @@ static void log_mode_after_vcpu_run(struct kvm_vcpu *vcpu, int ret, int err)
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mode->after_vcpu_run(vcpu, ret, err);
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}
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static void log_mode_before_vcpu_join(void)
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{
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struct log_mode *mode = &log_modes[host_log_mode];
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if (mode->before_vcpu_join)
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mode->before_vcpu_join();
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}
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static void generate_random_array(uint64_t *guest_array, uint64_t size)
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{
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uint64_t i;
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@ -719,6 +706,7 @@ static void run_test(enum vm_guest_mode mode, void *arg)
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struct kvm_vm *vm;
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unsigned long *bmap;
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uint32_t ring_buf_idx = 0;
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int sem_val;
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if (!log_mode_supported()) {
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print_skip("Log mode '%s' not supported",
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@ -788,12 +776,22 @@ static void run_test(enum vm_guest_mode mode, void *arg)
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/* Start the iterations */
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iteration = 1;
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sync_global_to_guest(vm, iteration);
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host_quit = false;
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WRITE_ONCE(host_quit, false);
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host_dirty_count = 0;
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host_clear_count = 0;
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host_track_next_count = 0;
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WRITE_ONCE(dirty_ring_vcpu_ring_full, false);
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/*
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* Ensure the previous iteration didn't leave a dangling semaphore, i.e.
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* that the main task and vCPU worker were synchronized and completed
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* verification of all iterations.
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*/
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sem_getvalue(&sem_vcpu_stop, &sem_val);
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TEST_ASSERT_EQ(sem_val, 0);
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sem_getvalue(&sem_vcpu_cont, &sem_val);
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TEST_ASSERT_EQ(sem_val, 0);
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pthread_create(&vcpu_thread, NULL, vcpu_worker, vcpu);
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while (iteration < p->iterations) {
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@ -819,15 +817,21 @@ static void run_test(enum vm_guest_mode mode, void *arg)
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assert(host_log_mode == LOG_MODE_DIRTY_RING ||
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atomic_read(&vcpu_sync_stop_requested) == false);
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vm_dirty_log_verify(mode, bmap);
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sem_post(&sem_vcpu_cont);
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iteration++;
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/*
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* Set host_quit before sem_vcpu_cont in the final iteration to
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* ensure that the vCPU worker doesn't resume the guest. As
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* above, the dirty ring test may stop and wait even when not
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* explicitly request to do so, i.e. would hang waiting for a
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* "continue" if it's allowed to resume the guest.
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*/
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if (++iteration == p->iterations)
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WRITE_ONCE(host_quit, true);
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sem_post(&sem_vcpu_cont);
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sync_global_to_guest(vm, iteration);
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}
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/* Tell the vcpu thread to quit */
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host_quit = true;
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log_mode_before_vcpu_join();
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pthread_join(vcpu_thread, NULL);
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pr_info("Total bits checked: dirty (%"PRIu64"), clear (%"PRIu64"), "
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