Re: [virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for big packets

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 09, 2022 at 04:24:23PM -0700, Si-Wei Liu wrote:
> 
> 
> On 8/9/2022 3:49 PM, Parav Pandit wrote:
> > > From: Michael S. Tsirkin <mst@redhat.com>
> > > Sent: Tuesday, August 9, 2022 6:26 PM
> > > To: Parav Pandit <parav@nvidia.com>
> > > Cc: Si-Wei Liu <si-wei.liu@oracle.com>; Jason Wang
> > > <jasowang@redhat.com>; Gavin Li <gavinl@nvidia.com>; Hemminger,
> > > Stephen <stephen@networkplumber.org>; davem
> > > <davem@davemloft.net>; virtualization <virtualization@lists.linux-
> > > foundation.org>; Virtio-Dev <virtio-dev@lists.oasis-open.org>;
> > > jesse.brandeburg@intel.com; alexander.h.duyck@intel.com;
> > > kubakici@wp.pl; sridhar.samudrala@intel.com; loseweigh@gmail.com; Gavi
> > > Teitz <gavi@nvidia.com>
> > > Subject: Re: [virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for
> > > big packets
> > > 
> > > On Tue, Aug 09, 2022 at 09:49:03PM +0000, Parav Pandit wrote:
> > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > Sent: Tuesday, August 9, 2022 5:38 PM
> > > > [..]
> > > > > > > I think virtio-net driver doesn't differentiate MTU and MRU, in
> > > > > > > which case the receive buffer will be reduced to fit the 1500B
> > > > > > > payload size when mtu is lowered down to 1500 from 9000.
> > > > > > How? Driver reduced the mXu to 1500, say it is improved to post
> > > > > > buffers of
> > > > > 1500 bytes.
> > > > > > Device doesn't know about it because mtu in config space is RO field.
> > > > > > Device keep dropping 9K packets because buffers posted are 1500
> > > bytes.
> > > > > > This is because device follows the spec " The device MUST NOT pass
> > > > > received packets that exceed mtu".
> > > > > 
> > > > > 
> > > > > The "mtu" here is the device config field, which is
> > > > > 
> > > > >          /* Default maximum transmit unit advice */
> > > > > 
> > > > It is the field from struct virtio_net_config.mtu. right?
> > > > This is RO field for driver.
> > > > 
> > > > > there is no guarantee device will not get a bigger packet.
> > > > Right. That is what I also hinted.
> > > > Hence, allocating buffers worth upto mtu is safer.
> > > yes
> > > 
> > > > When user overrides it, driver can be further optimized to honor such new
> > > value on rx buffer posting.
> > > 
> > > no, not without a feature bit promising device won't get wedged.
> > > 
> > I mean to say as_it_stands today, driver can decide to post smaller buffers with larger mtu.
> > Why device should be affected with it?
> > ( I am not proposing such weird configuration but asking for sake of correctness).
> I am also confused how the device can be wedged in this case.

Yea sorry. I misunderstood the code. It can't be.

-- 
MST