Jason Wang
2022-Aug-10 06:59 UTC
[virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for big packets
On Wed, Aug 10, 2022 at 2:15 PM Michael S. Tsirkin <mst at redhat.com> wrote:> > 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 at redhat.com> > > > > > Sent: Tuesday, August 9, 2022 6:26 PM > > > > > To: Parav Pandit <parav at nvidia.com> > > > > > Cc: Si-Wei Liu <si-wei.liu at oracle.com>; Jason Wang > > > > > <jasowang at redhat.com>; Gavin Li <gavinl at nvidia.com>; Hemminger, > > > > > Stephen <stephen at networkplumber.org>; davem > > > > > <davem at davemloft.net>; virtualization <virtualization at lists.linux- > > > > > foundation.org>; Virtio-Dev <virtio-dev at lists.oasis-open.org>; > > > > > jesse.brandeburg at intel.com; alexander.h.duyck at intel.com; > > > > > kubakici at wp.pl; sridhar.samudrala at intel.com; loseweigh at gmail.com; Gavi > > > > > Teitz <gavi at 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 at 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?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> > > -- > > MST >
Michael S. Tsirkin
2022-Aug-10 09:03 UTC
[virtio-dev] [PATCH] virtio-net: use mtu size as buffer length for big packets
On Wed, Aug 10, 2022 at 02:59:55PM +0800, Jason Wang wrote:> On Wed, Aug 10, 2022 at 2:15 PM Michael S. Tsirkin <mst at redhat.com> wrote: > > > > 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 at redhat.com> > > > > > > Sent: Tuesday, August 9, 2022 6:26 PM > > > > > > To: Parav Pandit <parav at nvidia.com> > > > > > > Cc: Si-Wei Liu <si-wei.liu at oracle.com>; Jason Wang > > > > > > <jasowang at redhat.com>; Gavin Li <gavinl at nvidia.com>; Hemminger, > > > > > > Stephen <stephen at networkplumber.org>; davem > > > > > > <davem at davemloft.net>; virtualization <virtualization at lists.linux- > > > > > > foundation.org>; Virtio-Dev <virtio-dev at lists.oasis-open.org>; > > > > > > jesse.brandeburg at intel.com; alexander.h.duyck at intel.com; > > > > > > kubakici at wp.pl; sridhar.samudrala at intel.com; loseweigh at gmail.com; Gavi > > > > > > Teitz <gavi at 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 at 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? > > The advantage of this is to keep TX working. Or we can use device > reset as a fallback if there's no vq reset. > > ThanksDevice reset is really annoying in that it loses all the state: rx filters etc etc.> > > > > > -- > > > MST > >