On Friday 28 November 2008 00:27:05 Mark McLoughlin wrote:
> Hi Rusty,
>
> On Thu, 2008-11-27 at 23:00 +1030, Rusty Russell wrote:
> > On Thursday 27 November 2008 00:28:11 Mark McLoughlin wrote:
> > > We don't really have a max tx packet size limit, so allow configuring
> > > the device with up to 64k tx MTU.
> >
> > Hi Mark,
> >
> > Just one comment: maybe we should be conservative and maybe limit to 1500
> > if the host doesn't offer any of the GSO or MRG_RXBUF features?
>
> That was actually what I was going to do until I thought about it a bit
> more and discussed it with Herbert.
>
> The virtio_net MTU only affects the transmit path, so there shouldn't be
> any issue with a host that doesn't support those features.
Not quite what I meant. A minimal host can reasonably expect ethernet-fitting
packets. If it supports GSO of course it must handle larger ones. Otherwise
we should add YA feature bit or even a max-mtu field.
So, I was thinking something like this over your patch (I also removed the
used-once MAX and MIN definitions; I dislike gratuitous indirection):
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -480,26 +480,24 @@ static struct ethtool_ops virtnet_ethtoo
.set_sg = ethtool_op_set_sg,
};
-#define MIN_MTU 68
-#define MAX_MTU 65535
-
static int virtnet_change_mtu(struct net_device *dev, int new_mtu)
{
struct virtnet_info *vi = netdev_priv(dev);
int max_mtu;
- /* Only allow a large MTU if we know we have a chance
- * of also supporting that MTU on the receive side. */
- if (vi->mergeable_rx_bufs || vi->big_packets)
- max_mtu = MAX_MTU;
+ /* A host which can handle GSO must handle large packets. */
+ if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_GSO)
+ || virtio_has_feature(vi->vdev, VIRTIO_NET_F_HOST_TSO4)
+ || virtio_has_feature(vi->vdev, VIRTIO_NET_F_HOST_TSO6)
+ || virtio_has_feature(vi->vdev, VIRTIO_NET_F_HOST_UFO))
+ max_mtu = 65535;
else
max_mtu = ETH_DATA_LEN;
- if (new_mtu < MIN_MTU || new_mtu > max_mtu)
+ if (new_mtu < 68 || new_mtu > max_mtu)
return -EINVAL;
dev->mtu = new_mtu;
-
return 0;
}
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