Re: [PATCH v2 2/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API

[Stefan Hajnoczi]

On Fri, May 07, 2021 at 07:06:34PM +0200, Philippe Mathieu-Daudé wrote:
> By directly using find_first_bit() and find_next_bit from the
> "bitops.h" API to iterate over the bitmap, we can remove the
> bitmap[] variable-length array copy on the stack and the complex
> manual bit testing/clearing logic.
> 
> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> Suggested-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  hw/block/dataplane/virtio-blk.c | 19 ++++---------------
>  1 file changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index e9050c8987e..a7b5bda06fc 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -60,23 +60,12 @@ static void notify_guest_bh(void *opaque)
>  {
>      VirtIOBlockDataPlane *s = opaque;
>      unsigned nvqs = s->conf->num_queues;
> -    unsigned long bitmap[BITS_TO_LONGS(nvqs)];
> -    unsigned j;
>  
> -    memcpy(bitmap, s->batch_notify_vqs, sizeof(bitmap));
> -    memset(s->batch_notify_vqs, 0, sizeof(bitmap));
> +    for (unsigned long i = find_first_bit(s->batch_notify_vqs, nvqs);
> +             i < nvqs; i = find_next_bit(s->batch_notify_vqs, nvqs, i)) {
> +        VirtQueue *vq = virtio_get_queue(s->vdev, i);
>  
> -    for (j = 0; j < nvqs; j += BITS_PER_LONG) {
> -        unsigned long bits = bitmap[j / BITS_PER_LONG];
> -
> -        while (bits != 0) {
> -            unsigned i = j + ctzl(bits);
> -            VirtQueue *vq = virtio_get_queue(s->vdev, i);
> -
> -            virtio_notify_irqfd(s->vdev, vq);
> -
> -            bits &= bits - 1; /* clear right-most bit */
> -        }
> +        virtio_notify_irqfd(s->vdev, vq);
>      }
>  }

Bits in s->batch_notify_vqs[] must be cleared. Otherwise we may raise
spurious irqs next time this function is called. The memset() can be
moved after the loop.

Stefan

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