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

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Parav Pandit <parav@nvidia.com>
Cc: "alexander.h.duyck@intel.com" <alexander.h.duyck@intel.com>,
	Virtio-Dev <virtio-dev@lists.oasis-open.org>,
	"kubakici@wp.pl" <kubakici@wp.pl>,
	"sridhar.samudrala@intel.com" <sridhar.samudrala@intel.com>,
	"jesse.brandeburg@intel.com" <jesse.brandeburg@intel.com>,
	Gavi Teitz <gavi@nvidia.com>,
	virtualization <virtualization@lists.linux-foundation.org>,
	"Hemminger, Stephen" <stephen@networkplumber.org>,
	"loseweigh@gmail.com" <loseweigh@gmail.com>,
	davem <davem@davemloft.net>, Gavin Li <gavinl@nvidia.com>
Subject: Re: [virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for big packets
Date: Wed, 10 Aug 2022 13:12:21 -0400	[thread overview]
Message-ID: <20220810131209-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <PH0PR12MB54811BD79FAFFAFF38231D38DC659@PH0PR12MB5481.namprd12.prod.outlook.com>

On Wed, Aug 10, 2022 at 05:06:33PM +0000, Parav Pandit wrote:
> 
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, August 10, 2022 12:59 PM
> > 
> > On Wed, Aug 10, 2022 at 04:22:41PM +0000, Parav Pandit wrote:
> > >
> > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > Sent: Wednesday, August 10, 2022 12:05 PM
> > > >
> > > > On Wed, Aug 10, 2022 at 04:00:08PM +0000, Parav Pandit wrote:
> > > > >
> > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > Sent: Wednesday, August 10, 2022 5:03 AM
> > > > > > > >
> > > > > > > > Should we make this depend on the vq reset ability maybe?
> > > > > > >
> > > > > > > The advantage of this is to keep TX working. Or we can use
> > > > > > > device reset as a fallback if there's no vq reset.
> > > > > > >
> > > > > > > Thanks
> > > > > >
> > > > > > Device reset is really annoying in that it loses all the state:
> > > > > > rx filters etc etc.
> > > > >
> > > > > The elegant solution is let driver tell the new mtu to the device.
> > > > > One way to do so is by using existing ctrl vq.
> > > >
> > > > That will need a new feature bit.
> > > >
> > > Yes. ctrl vq can tell what all configuration does it allow. :) Or you
> > > prefer feature bit?
> > 
> > We did feature bits for this in the past.
> > 
> Ok. Will try to draft the update for future.
> 
> Gavin should repost the patch to address comments unrelated to this future bit anyway.
> Right?

Right.

> > > > > If merged buffer is done, and new mtu is > minimum posting size,
> > > > > no need
> > > > to undergo vq reset.
> > > > > If merged buffer is not done, and buffer posted are smaller than
> > > > > new mtu,
> > > > undergo vq reset optionally.
> > > >
> > > > This can be done with or without sending mtu to device.
> > > Yes, telling mtu to device helps device to optimize and adhere to the spec
> > line " The device MUST NOT pass received packets that exceed mtu" in
> > section 5.1.4.1.
> > 
> > Again, that line refers to \field{mtu} which is the max mtu supported,
> > irrespective to anything driver does.
> > 
> > --
> > MST

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