Re: [PATCH 10/20] KVM: selftests: Keep dirty_log_test vCPU in guest until it needs to stop

Maxim Levitsky
2024-12-18T00:01:12Z

On Fri, 2024-12-13 at 17:07 -0800, Sean Christopherson wrote:
> In the dirty_log_test guest code, exit to userspace

Once again, "exit to userspace" is misleading.

> only when the vCPU is
> explicitly told to stop. Periodically exiting just to check if a flag has
> been set is unnecessary, weirdly complex, and wastes time handling exits
> that could be used to dirty memory.

>
> Opportunistically convert 'i' to a uint64_t to guard against the unlikely
> scenario that guest_num_pages exceeds the storage of an int.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> tools/testing/selftests/kvm/dirty_log_test.c | 43 ++++++++++----------
> 1 file changed, 22 insertions(+), 21 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
> index 8d31e275a23d..40c8f5551c8e 100644
> --- a/tools/testing/selftests/kvm/dirty_log_test.c
> +++ b/tools/testing/selftests/kvm/dirty_log_test.c
> @@ -31,9 +31,6 @@
> /* Default guest test virtual memory offset */
> #define DEFAULT_GUEST_TEST_MEM 0xc0000000
>
> -/* How many pages to dirty for each guest loop */
> -#define TEST_PAGES_PER_LOOP 1024
> -
> /* How many host loops to run (one KVM_GET_DIRTY_LOG for each loop) */
> #define TEST_HOST_LOOP_N 32UL
>
> @@ -75,6 +72,7 @@ static uint64_t host_page_size;
> static uint64_t guest_page_size;
> static uint64_t guest_num_pages;
> static uint64_t iteration;
> +static bool vcpu_stop;
>
> /*
> * Guest physical memory offset of the testing memory slot.
> @@ -96,9 +94,10 @@ static uint64_t guest_test_virt_mem = DEFAULT_GUEST_TEST_MEM;
> static void guest_code(void)
> {
> uint64_t addr;
> - int i;
>
> #ifdef __s390x__
> + uint64_t i;
> +
> /*
> * On s390x, all pages of a 1M segment are initially marked as dirty
> * when a page of the segment is written to for the very first time.
> @@ -112,7 +111,7 @@ static void guest_code(void)
> #endif
>
> while (true) {
> - for (i = 0; i < TEST_PAGES_PER_LOOP; i++) {
> + while (!READ_ONCE(vcpu_stop)) {
> addr = guest_test_virt_mem;
> addr += (guest_random_u64(&guest_rng) % guest_num_pages)
> * guest_page_size;
> @@ -140,14 +139,7 @@ static uint64_t host_track_next_count;
> /* Whether dirty ring reset is requested, or finished */
> static sem_t sem_vcpu_stop;
> static sem_t sem_vcpu_cont;
> -/*
> - * This is only set by main thread, and only cleared by vcpu thread. It is
> - * used to request vcpu thread to stop at the next GUEST_SYNC, since GUEST_SYNC
> - * is the only place that we'll guarantee both "dirty bit" and "dirty data"
> - * will match. E.g., SIG_IPI won't guarantee that if the vcpu is interrupted
> - * after setting dirty bit but before the data is written.
> - */
> -static atomic_t vcpu_sync_stop_requested;
> +
> /*
> * This is updated by the vcpu thread to tell the host whether it's a
> * ring-full event. It should only be read until a sem_wait() of
> @@ -272,9 +264,7 @@ static void clear_log_collect_dirty_pages(struct kvm_vcpu *vcpu, int slot,
> /* Should only be called after a GUEST_SYNC */
> static void vcpu_handle_sync_stop(void)
> {
> - if (atomic_read(&vcpu_sync_stop_requested)) {
> - /* It means main thread is sleeping waiting */
> - atomic_set(&vcpu_sync_stop_requested, false);
> + if (READ_ONCE(vcpu_stop)) {
> sem_post(&sem_vcpu_stop);
> sem_wait(&sem_vcpu_cont);
> }
> @@ -801,11 +791,24 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> }
>
> /*
> - * See vcpu_sync_stop_requested definition for details on why
> - * we need to stop vcpu when verify data.
> + * Stop the vCPU prior to collecting and verifying the dirty
> + * log. If the vCPU is allowed to run during collection, then
> + * pages that are written during this iteration may be missed,
> + * i.e. collected in the next iteration. And if the vCPU is
> + * writing memory during verification, pages that this thread
> + * sees as clean may be written with this iteration's value.
> */
> - atomic_set(&vcpu_sync_stop_requested, true);
> + WRITE_ONCE(vcpu_stop, true);
> + sync_global_to_guest(vm, vcpu_stop);
> sem_wait(&sem_vcpu_stop);
> +
> + /*
> + * Clear vcpu_stop after the vCPU thread has acknowledge the
> + * stop request and is waiting, i.e. is definitely not running!
> + */
> + WRITE_ONCE(vcpu_stop, false);
> + sync_global_to_guest(vm, vcpu_stop);
> +
> /*
> * NOTE: for dirty ring, it's possible that we didn't stop at
> * GUEST_SYNC but instead we stopped because ring is full;
> @@ -813,8 +816,6 @@ static void run_test(enum vm_guest_mode mode, void *arg)
> * the flush of the last page, and since we handle the last
> * page specially verification will succeed anyway.
> */
> - assert(host_log_mode == LOG_MODE_DIRTY_RING ||
> - atomic_read(&vcpu_sync_stop_requested) == false);
> vm_dirty_log_verify(mode, bmap);
>
> /*