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

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

On Wed, Aug 10, 2022 at 02:14:07AM -0400, Michael S. Tsirkin wrote:
> 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.

Here's a problem as I see it. Let's say we reduce mtu.
Small buffers are added. Now we increase mtu.
Device will drop all packets until small buffers are consumed.

Should we make this depend on the vq reset ability maybe?

> -- 
> MST