linux-kernel - Re: [PATCH] virtio_net: enable big packets for large MTU values

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Message-ID: <5304103e-5a0b-230b-6bd0-1b33ab546889@redhat.com>
Date:   Thu, 30 Mar 2017 12:03:36 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH] virtio_net: enable big packets for large MTU values

On 2017年03月29日 20:38, Michael S. Tsirkin wrote:
> If one enables e.g. jumbo frames without mergeable
> buffers, packets won't fit in 1500 byte buffers
> we use. Switch to big packet mode instead.
> TODO: make sizing more exact, possibly extend small
> packet mode to use larger pages.
>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>   drivers/net/virtio_net.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index e0fb3707..9dc31dc 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2428,6 +2428,10 @@ static int virtnet_probe(struct virtio_device *vdev)
>   			dev->mtu = mtu;
>   			dev->max_mtu = mtu;
>   		}
> +
> +		/* TODO: size buffers correctly in this case. */
> +		if (dev->mtu > ETH_DATA_LEN)
> +			vi->big_packets = true;
>   	}
>   
>   	if (vi->any_header_sg)

Ok, I think maybe we should fail the probe of small buffer in this case 
and decrease the max_mtu to ETH_DATA_LEN if small buffer is used (since 
user are allowed to increase the mtu).

Thanks