Received-SPF: Pass (protection.outlook.com: domain of nvidia.com designates
 216.228.112.34 as permitted sender) receiver=protection.outlook.com;
 client-ip=216.228.112.34; helo=mail.nvidia.com;
Date:   Thu, 25 Nov 2021 09:20:40 +0200
From:   Eli Cohen <elic@nvidia.com>
To:     Jason Wang <jasowang@redhat.com>
CC:     mst <mst@redhat.com>,
        virtualization <virtualization@lists.linux-foundation.org>,
        netdev <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH net] virtio-net: enable big mode correctly
Message-ID: <20211125072040.GA213301@mtl-vdi-166.wap.labs.mlnx>
References: <20211125060547.11961-1-jasowang@redhat.com>
 <20211125070939.GC211101@mtl-vdi-166.wap.labs.mlnx>
 <CACGkMEsNsQ_XWTvdjaCEdo8sYaLew24zU1UUCJrokM-Koxj4fw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Disposition: inline
In-Reply-To: <CACGkMEsNsQ_XWTvdjaCEdo8sYaLew24zU1UUCJrokM-Koxj4fw@mail.gmail.com>
User-Agent: Mutt/1.9.5 (bf161cf53efb) (2018-04-13)
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 25 Nov 2021 07:20:46.5291
 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: eccd31f8-51db-4d32-fc90-08d9afe41997
X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a
X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.112.34];Helo=[mail.nvidia.com]
X-MS-Exchange-CrossTenant-AuthSource: CO1NAM11FT031.eop-nam11.prod.protection.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Anonymous
X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR12MB4500
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Nov 25, 2021 at 03:15:33PM +0800, Jason Wang wrote:
> On Thu, Nov 25, 2021 at 3:09 PM Eli Cohen <elic@nvidia.com> wrote:
> >
> > On Thu, Nov 25, 2021 at 02:05:47PM +0800, Jason Wang wrote:
> > > When VIRTIO_NET_F_MTU feature is not negotiated, we assume a very
> > > large max_mtu. In this case, using small packet mode is not correct
> > > since it may breaks the networking when MTU is grater than
> > > ETH_DATA_LEN.
> > >
> > > To have a quick fix, simply enable the big packet mode when
> > > VIRTIO_NET_F_MTU is not negotiated. We can do optimization on top.
> > >
> > > Reported-by: Eli Cohen <elic@nvidia.com>
> > > Cc: Eli Cohen <elic@nvidia.com>
> > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > ---
> > >  drivers/net/virtio_net.c | 7 ++++---
> > >  1 file changed, 4 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 7c43bfc1ce44..83ae3ef5eb11 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -3200,11 +3200,12 @@ 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;
> > >       }
> > >
> > > +     /* TODO: size buffers correctly in this case. */
> > > +     if (dev->max_mtu > ETH_DATA_LEN)
> > > +             vi->big_packets = true;
> > > +
> >
> > If VIRTIO_NET_F_MTU is provided, then dev->max_mtu is going to equal
> > ETH_DATA_LEN (will be set in ether_setup()) so I don't think it will set
> > big_packets to true.
> 
> I may miss something, the dev->max_mtu is just assigned to the mtu
> value read from the config space in the code block above  (inside the
> feature check of VIRTIO_NET_F_MTU).

Sorry, I meant "If VIRTIO_NET_F_MTU is ***NOT*** provided". In that case
dev->max_mtu eauals ETH_DATA_LEN so you won't set vi->big_packets to
true.

> 
> Thanks
> 
> >
> >
> > >       if (vi->any_header_sg)
> > >               dev->needed_headroom = vi->hdr_len;
> > >
> > > --
> > > 2.25.1
> > >
> >
>