Re: [PATCH 17/20] KVM: selftests: Tighten checks around prev iter's last dirty page in ring

[Maxim Levitsky <mlevitsk@redhat.com>]

On Fri, 2024-12-13 at 17:07 -0800, Sean Christopherson wrote:
> Now that each iteration collects all dirty entries and ensures the guest
> *completes* at least one write, tighten the exemptions for the last dirty
> page of the previous iteration.  Specifically, the only legal value (other
> than the current iteration) is N-1.
> 
> Unlike the last page for the current iteration, the in-progress write from
> the previous iteration is guaranteed to have completed, otherwise the test
> would have hung.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  tools/testing/selftests/kvm/dirty_log_test.c | 22 +++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/dirty_log_test.c b/tools/testing/selftests/kvm/dirty_log_test.c
> index 8eb51597f762..18d41537e737 100644
> --- a/tools/testing/selftests/kvm/dirty_log_test.c
> +++ b/tools/testing/selftests/kvm/dirty_log_test.c
> @@ -517,14 +517,22 @@ static void vm_dirty_log_verify(enum vm_guest_mode mode, unsigned long **bmap)
>  
>  			if (host_log_mode == LOG_MODE_DIRTY_RING) {
>  				/*
> -				 * The last page in the ring from this iteration
> -				 * or the previous can be written with the value
> -				 * from the previous iteration (relative to the
> -				 * last page's iteration), as the value to be
> -				 * written may be cached in a CPU register.
> +				 * The last page in the ring from previous
> +				 * iteration can be written with the value
> +				 * from the previous iteration, as the value to
> +				 * be written may be cached in a CPU register.
>  				 */
> -				if ((page == dirty_ring_last_page ||
> -				     page == dirty_ring_prev_iteration_last_page) &&
> +				if (page == dirty_ring_prev_iteration_last_page &&
> +				    val == iteration - 1)
> +					continue;
> +
> +				/*
> +				 * Any value from a previous iteration is legal
> +				 * for the last entry, as the write may not yet
> +				 * have retired, i.e. the page may hold whatever
> +				 * it had before this iteration started.
> +				 */
> +				if (page == dirty_ring_last_page &&
>  				    val < iteration)
>  					continue;
>  			} else if (!val && iteration == 1 && bmap0_dirty) {

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>