On Tue, Aug 09, 2022 at 10:49:48PM +0000, Parav Pandit wrote: > > From: Michael S. Tsirkin <mst@xxxxxxxxxx> > > Sent: Tuesday, August 9, 2022 6:26 PM > > To: Parav Pandit <parav@xxxxxxxxxx> > > Cc: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>; Jason Wang > > <jasowang@xxxxxxxxxx>; Gavin Li <gavinl@xxxxxxxxxx>; Hemminger, > > Stephen <stephen@xxxxxxxxxxxxxxxxxx>; davem > > <davem@xxxxxxxxxxxxx>; virtualization <virtualization@lists.linux- > > foundation.org>; Virtio-Dev <virtio-dev@xxxxxxxxxxxxxxxxxxxx>; > > jesse.brandeburg@xxxxxxxxx; alexander.h.duyck@xxxxxxxxx; > > kubakici@xxxxx; sridhar.samudrala@xxxxxxxxx; loseweigh@xxxxxxxxx; Gavi > > Teitz <gavi@xxxxxxxxxx> > > 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@xxxxxxxxxx> > > > > 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). They just are because drivers did not do this. > > > > And there is no guarantee such a packet will be dropped as opposed > > > > to wedging the device if userspace insists on adding smaller buffers. > > > > > > > If user space insists on small buffers, so be it. > > > > If previously things worked, the "so be it" is a regression and blaming users > > won't help us. > > > I am not suggesting above. > This was Si-Wei's suggestion that somehow driver wants to post smaller buffers than the mtu because user knows what peer is doing. > So may be driver can be extended to give more weight on user config. > > > > It only works when user exactly know what user is doing in the whole > > network. > > > > If you want to claim this you need a new feature bit. > > > Why is a new bit needed to tell device? > User is doing something its own config mismatching the buffers and mtu. > A solid use case hasn't emerged for this yet. > > If user wants to modify the mtu, we should just make virtio_net_config.mtu as RW field using new feature bit. > Is that what you mean? > If so, yes, it makes things very neat where driver and device are aligned to each other, the way they are today. > Only limitation is that its one-way. = device tells to driver. > > > > When user prefers to override the device RO field, device is in the dark and > > things work on best effort basis. > > > > Dropping packets is best effort. Getting stuck forever isn't, that's a quality of > > implementation issue. > > > Not sure, why things get stuck for ever. Maybe you have example to explain. > I am mostly missing something. I sent an explanation a bit earlier. It's more or less a bug. > > > This must be a reasonably advance user who has good knowledge of its > > network topology etc. > > > > > > For such case, may be yes, driver should be further optimized. > > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
