Re: [Qemu-devel] [PATCH RFC] virtio: add virtqueue_fill_partial

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/04/2015 16:18, Michael S. Tsirkin wrote:
> +/*
> + * Some devices dirty guest memory but don't want to tell guest about it. In
> + * that case, use virtqueue_fill_partial: host_len is >= the amount of guest
> + * memory actually written, guest_len is how much we guarantee to guest.
> + * If you know exactly how much was written, use virtqueue_fill instead.

"If you never leave holes unwritten in the iov, use virtqueue_fill
instead.  If the guest is not relying on iov boundaries, it should never
be necessary to use this function."

The API looks okay though.

Paolo

> + */
> +void virtqueue_fill_partial(VirtQueue *vq, const VirtQueueElement *elem,
> +                            unsigned int host_len, unsigned int guest_len,
> +                            unsigned int idx)
>  {
>      unsigned int offset;
>      int i;
>  
> -    trace_virtqueue_fill(vq, elem, len, idx);
> +    assert(host_len >= guest_len);
> +
> +    trace_virtqueue_fill(vq, elem, guest_len, idx);
>  
>      offset = 0;
>      for (i = 0; i < elem->in_num; i++) {
> -        size_t size = MIN(len - offset, elem->in_sg[i].iov_len);
> +        size_t size = MIN(host_len - offset, elem->in_sg[i].iov_len);
>  
>          cpu_physical_memory_unmap(elem->in_sg[i].iov_base,
>                                    elem->in_sg[i].iov_len,
> @@ -269,7 +278,13 @@ void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
>  
>      /* Get a pointer to the next entry in the used ring. */
>      vring_used_ring_id(vq, idx, elem->index);
> -    vring_used_ring_len(vq, idx, len);
> +    vring_used_ring_len(vq, idx, guest_len);
> +}
> +