Re: [PATCH] virtio: increase VIRTQUEUE_MAX_SIZE to 32k

[Michael S. Tsirkin]

On Sun, Oct 03, 2021 at 08:15:36PM +0200, Christian Schoenebeck wrote:
> VIRTQUEUE_MAX_SIZE reflects the absolute theoretical maximum
> queue size possible, which is actually the maximum queue size
> allowed by the virtio protocol. The appropriate value for
> VIRTQUEUE_MAX_SIZE is therefore 32768:
> 
> https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-240006
> 
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

Problem is this then exceeds UIO_MAXIOV and e.g. virtio net
assumes that an iovec it gets from guest can be passed directly
to linux. Either we need to remove that restriction
(e.g. by doing an extra copy if iov size is bigger)
or add the limitation in net-specific code. blk and scsi
might be affected too, but these have a per-device
limit which can be tweaked.

> ---
>  include/hw/virtio/virtio.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index 8bab9cfb75..1f18efa0bc 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -48,7 +48,7 @@ size_t virtio_feature_get_config_size(const VirtIOFeature 
> *features,
>  
>  typedef struct VirtQueue VirtQueue;
>  
> -#define VIRTQUEUE_MAX_SIZE 1024
> +#define VIRTQUEUE_MAX_SIZE 32768
>  
>  typedef struct VirtQueueElement
>  {
> -- 
> 2.20.1