Re: [Qemu-devel] [PATCH 05/10] virtio: slim down allocation of VirtQueueElements

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Fri, 15 Jan 2016 13:41:53 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> Build the addresses and s/g lists on the stack, and then copy them
> to a VirtQueueElement that is just as big as required to contain this
> particular s/g list.  The cost of the copy is minimal compared to that
> of a large malloc.
> 
> When virtqueue_map is used on the destination side of migration or on
> loadvm, the iovecs have already been split at memory region boundary,
> so we can just reuse the out_num/in_num we find in the file.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  hw/virtio/virtio.c | 82 +++++++++++++++++++++++++++++++++---------------------
>  1 file changed, 51 insertions(+), 31 deletions(-)
> 

>      /* Collect all the descriptors */
>      do {
> -        struct iovec *sg;
> +        hwaddr pa = vring_desc_addr(vdev, desc_pa, i);
> +        size_t len = vring_desc_len(vdev, desc_pa, i);
> 
>          if (vring_desc_flags(vdev, desc_pa, i) & VRING_DESC_F_WRITE) {
> -            if (elem->in_num >= VIRTQUEUE_MAX_SIZE) {
> -                error_report("Too many write descriptors in indirect table");
> -                exit(1);
> -            }
> -            elem->in_addr[elem->in_num] = vring_desc_addr(vdev, desc_pa, i);
> -            sg = &elem->in_sg[elem->in_num++];
> +            virtqueue_map_desc(&in_num, addr + out_num, iov + out_num,
> +                               VIRTQUEUE_MAX_SIZE - out_num, 1, pa, len);

Minor nit: I'd probably use 'true' here...

>          } else {
> -            if (elem->out_num >= VIRTQUEUE_MAX_SIZE) {
> -                error_report("Too many read descriptors in indirect table");
> +            if (in_num) {
> +                error_report("Incorrect order for descriptors");
>                  exit(1);
>              }
> -            elem->out_addr[elem->out_num] = vring_desc_addr(vdev, desc_pa, i);
> -            sg = &elem->out_sg[elem->out_num++];
> +            virtqueue_map_desc(&out_num, addr, iov,
> +                               VIRTQUEUE_MAX_SIZE, 0, pa, len);

...and 'false' here.

>          }
> 
> -        sg->iov_len = vring_desc_len(vdev, desc_pa, i);
> -
>          /* If we've got too many, that implies a descriptor loop. */
> -        if ((elem->in_num + elem->out_num) > max) {
> +        if ((in_num + out_num) > max) {
>              error_report("Looped descriptor");
>              exit(1);
>          }
>      } while ((i = virtqueue_next_desc(vdev, desc_pa, i, max)) != max);

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>