Re: [PATCH v2] virtio: use zeroed memory for simple TX header

["Tan, Jianfeng" <jianfeng.tan@intel.com>]

Hi,

On 4/5/2016 10:11 AM, Rich Lane wrote:
> For simple TX the virtio-net header must be zeroed, but it was using memory
> that had been initialized with indirect descriptor tables. This resulted in
> "unsupported gso type" errors from librte_vhost.
>
> We can use the same memory for every descriptor to save cachelines in the
> vswitch.

Pointing all virtio_net_hdr into the same memory may brings performance, 
but how much? It also introduces difficulty to adding tso in future?

Thanks,
Jianfeng

>
> Fixes: 6dc5de3a (virtio: use indirect ring elements)
> Signed-off-by: Rich Lane <rlane@bigswitch.com>
> ---
> v1-v2:
> - Use offsetof to get address of tx_hdr
>
>   drivers/net/virtio/virtio_rxtx.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index 2b88efd..ef21d8e 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -377,7 +377,7 @@ virtio_dev_vring_start(struct virtqueue *vq, int queue_type)
>   				vq->vq_ring.desc[i + mid_idx].next = i;
>   				vq->vq_ring.desc[i + mid_idx].addr =
>   					vq->virtio_net_hdr_mem +
> -						i * vq->hw->vtnet_hdr_size;
> +					offsetof(struct virtio_tx_region, tx_hdr);
>   				vq->vq_ring.desc[i + mid_idx].len =
>   					vq->hw->vtnet_hdr_size;
>   				vq->vq_ring.desc[i + mid_idx].flags =