On Mon, May 08, 2023 at 03:41:56PM +0800, Xuan Zhuo wrote: > On Mon, 8 May 2023 02:43:24 -0400, "Michael S. Tsirkin" <m...@redhat.com> > wrote: > > On Mon, May 08, 2023 at 02:18:08PM +0800, Xuan Zhuo wrote: > > > On Mon, 8 May 2023 02:15:46 -0400, "Michael S. Tsirkin" <m...@redhat.com> > > > wrote: > > > > On Mon, May 08, 2023 at 10:01:59AM +0800, Xuan Zhuo wrote: > > > > > On Sun, 7 May 2023 04:58:58 -0400, "Michael S. Tsirkin" > > > > > <m...@redhat.com> wrote: > > > > > > On Sat, May 06, 2023 at 04:56:35PM +0800, Hao Chen wrote: > > > > > > > > > > > > > > > > > > > > > 在 2023/5/6 10:50, Xuan Zhuo 写道: > > > > > > > > On Sat, 6 May 2023 10:15:29 +0800, Hao Chen <ch...@yusur.tech> > > > > > > > > wrote: > > > > > > > > > When VIRTIO_NET_F_MTU(3) Device maximum MTU reporting is > > > > > > > > > supported. > > > > > > > > > If offered by the device, device advises driver about the > > > > > > > > > value of its > > > > > > > > > maximum MTU. If negotiated, the driver uses mtu as the maximum > > > > > > > > > MTU value. But there the driver also uses it as default mtu, > > > > > > > > > some devices may have a maximum MTU greater than 1500, this > > > > > > > > > may > > > > > > > > > cause some large packages to be discarded, > > > > > > > > > > > > > > > > You mean tx packet? > > > > > > > Yes. > > > > > > > > > > > > > > > > If yes, I do not think this is the problem of driver. > > > > > > > > > > > > > > > > Maybe you should give more details about the discard. > > > > > > > > > > > > > > > In the current code, if the maximum MTU supported by the virtio > > > > > > > net hardware > > > > > > > is 9000, the default MTU of the virtio net driver will also be > > > > > > > set to 9000. > > > > > > > When sending packets through "ping -s 5000", if the peer router > > > > > > > does not > > > > > > > support negotiating a path MTU through ICMP packets, the packets > > > > > > > will be > > > > > > > discarded. If the peer router supports negotiating path mtu > > > > > > > through ICMP > > > > > > > packets, the host side will perform packet sharding processing > > > > > > > based on the > > > > > > > negotiated path mtu, which is generally within 1500. > > > > > > > This is not a bugfix patch, I think setting the default mtu to > > > > > > > within 1500 > > > > > > > would be more suitable here.Thanks. > > > > > > > > > > > > I don't think VIRTIO_NET_F_MTU is appropriate for support for jumbo > > > > > > packets. > > > > > > The spec says: > > > > > > The device MUST forward transmitted packets of up to mtu (plus > > > > > > low level ethernet header length) size with > > > > > > gso_type NONE or ECN, and do so without fragmentation, after > > > > > > VIRTIO_NET_F_MTU has been success- > > > > > > fully negotiated. > > > > > > VIRTIO_NET_F_MTU has been designed for all kind of tunneling > > > > > > devices, > > > > > > and this is why we set mtu to max by default. > > > > > > > > > > > > For things like jumbo frames where MTU might or might not be > > > > > > available, > > > > > > a new feature would be more appropriate. > > > > > > > > > > > > > > > So for jumbo frame, what is the problem? > > > > > > > > > > We are trying to do this. @Heng > > > > > > > > > > Thanks. > > > > > > > > It is not a problem as such. But VIRTIO_NET_F_MTU will set the > > > > default MTU not just the maximum one, because spec seems to > > > > say it can. > > > > > > I see. > > > > > > In the case of Jumbo Frame, we also hope that the driver will set the > > > default > > > directly to the max mtu. Just like what you said "Bigger packets = better > > > performance." > > > > > > I don't know, in any scenario, when the hardware supports a large mtu, > > > but we do > > > not want the user to use it by default. > > > > When other devices on the same LAN have mtu set to 1500 and > > won't accept bigger packets. > > So, that depends on pmtu/tcp-probe-mtu. > > If the os without pmtu/tcp-probe-mtu has a bigger mtu, then it's big packet > will lost. > > Thanks. >
pmtu is designed for routing. LAN is supposed to be configured with a consistent MTU. > > > > > Of course, the scene that this patch > > > wants to handle does exist, but I have never thought that this is a > > > problem at > > > the driver level. > > > > > > Thanks. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > so I changed the MTU to a more > > > > > > > > > general 1500 when 'Device maximum MTU' bigger than 1500. > > > > > > > > > > > > > > > > > > Signed-off-by: Hao Chen <ch...@yusur.tech> > > > > > > > > > --- > > > > > > > > > drivers/net/virtio_net.c | 5 ++++- > > > > > > > > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > > > > > > > > > > > > > > > diff --git a/drivers/net/virtio_net.c > > > > > > > > > b/drivers/net/virtio_net.c > > > > > > > > > index 8d8038538fc4..e71c7d1b5f29 100644 > > > > > > > > > --- a/drivers/net/virtio_net.c > > > > > > > > > +++ b/drivers/net/virtio_net.c > > > > > > > > > @@ -4040,7 +4040,10 @@ static int virtnet_probe(struct > > > > > > > > > virtio_device *vdev) > > > > > > > > > goto free; > > > > > > > > > } > > > > > > > > > > > > > > > > > > - dev->mtu = mtu; > > > > > > > > > + if (mtu > 1500) > > > > > > > > > > > > > > > > s/1500/ETH_DATA_LEN/ > > > > > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > + dev->mtu = 1500; > > > > > > > > > + else > > > > > > > > > + dev->mtu = mtu; > > > > > > > > > dev->max_mtu = mtu; > > > > > > > > > } > > > > > > > > > > > > > > > > > > -- > > > > > > > > > 2.27.0 > > > > > > > > > > > > > > > > > > > > > _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
