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

[Maxim Levitsky <mlevitsk@redhat.com>]

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);
>  
>  		/*