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Message-ID: <1683531716.238961-1-xuanzhuo@linux.alibaba.com>
Subject: Re: [PATCH] virtio_net: set default mtu to 1500 when 'Device maximum MTU' bigger than 1500
Date:   Mon, 8 May 2023 15:41:56 +0800
From:   Xuan Zhuo <xuanzhuo@linux.alibaba.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     huangml@yusur.tech, zy@yusur.tech,
        Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "open list:VIRTIO CORE AND NET DRIVERS" 
        <virtualization@lists.linux-foundation.org>,
        "open list:NETWORKING DRIVERS" <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Hao Chen <chenh@yusur.tech>, hengqi@linux.alibaba.com
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On Mon, 8 May 2023 02:43:24 -0400, "Michael S. Tsirkin" <mst@redhat.com> wr=
ote:
> 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" <mst@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" <mst@redhat=
.com> wrote:
> > > > > On Sat, May 06, 2023 at 04:56:35PM +0800, Hao Chen wrote:
> > > > > >
> > > > > >
> > > > > > =E5=9C=A8 2023/5/6 10:50, Xuan Zhuo =E5=86=99=E9=81=93:
> > > > > > > On Sat,  6 May 2023 10:15:29 +0800, Hao Chen <chenh@yusur.tec=
h> wrote:
> > > > > > > > When VIRTIO_NET_F_MTU(3) Device maximum MTU reporting is su=
pported.
> > > > > > > > If offered by the device, device advises driver about the v=
alue of its
> > > > > > > > maximum MTU. If negotiated, the driver uses mtu as the maxi=
mum
> > > > > > > > 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 packet=
s will be
> > > > > > discarded. If the peer router supports negotiating path mtu thr=
ough 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 jum=
bo packets.
> > > > > The spec says:
> > > > > 	The device MUST forward transmitted packets of up to mtu (plus l=
ow level ethernet header length) size with
> > > > > 	gso_type NONE or ECN, and do so without fragmentation, after VIR=
TIO_NET_F_MTU has been success-
> > > > > 	fully negotiated.
> > > > > VIRTIO_NET_F_MTU has been designed for all kind of tunneling devi=
ces,
> > > > > and this is why we set mtu to max by default.
> > > > >
> > > > > For things like jumbo frames where MTU might or might not be avai=
lable,
> > > > > 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 d=
efault
> > directly to the max mtu. Just like what you said "Bigger packets =3D be=
tter
> > 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.


>
> > Of course, the scene that this patch
> > wants to handle does exist, but I have never thought that this is a pro=
blem 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 <chenh@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 virt=
io_device *vdev)
> > > > > > > >   			goto free;
> > > > > > > >   		}
> > > > > > > >
> > > > > > > > -		dev->mtu =3D mtu;
> > > > > > > > +		if (mtu > 1500)
> > > > > > >
> > > > > > > s/1500/ETH_DATA_LEN/
> > > > > > >
> > > > > > > Thanks.
> > > > > > >
> > > > > > > > +			dev->mtu =3D 1500;
> > > > > > > > +		else
> > > > > > > > +			dev->mtu =3D mtu;
> > > > > > > >   		dev->max_mtu =3D mtu;
> > > > > > > >   	}
> > > > > > > >
> > > > > > > > --
> > > > > > > > 2.27.0
> > > > > > > >
> > > > >
> > >
>