Re: [PATCH v2 2/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k

From: Christian Schoenebeck <qemu_oss@crudebyte.com>
To: qemu-devel@nongnu.org
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	"Kevin Wolf" <kwolf@redhat.com>,
	"Laurent Vivier" <lvivier@redhat.com>,
	qemu-block@nongnu.org, "Amit Shah" <amit@kernel.org>,
	"Jason Wang" <jasowang@redhat.com>,
	"David Hildenbrand" <david@redhat.com>,
	"Raphael Norwitz" <raphael.norwitz@nutanix.com>,
	virtio-fs@redhat.com, "Eric Auger" <eric.auger@redhat.com>,
	"Hanna Reitz" <hreitz@redhat.com>,
	"Gonglei (Arei)" <arei.gonglei@huawei.com>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	"Stefan Hajnoczi" <stefanha@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>,
	"Fam Zheng" <fam@euphon.net>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	"Greg Kurz" <groug@kaod.org>
Subject: Re: [PATCH v2 2/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k
Date: Tue, 05 Oct 2021 14:01:00 +0200	[thread overview]
Message-ID: <3198295.vc1OghKV7D@silver> (raw)
In-Reply-To: <20211005072243-mutt-send-email-mst@kernel.org>

On Dienstag, 5. Oktober 2021 13:24:36 CEST Michael S. Tsirkin wrote:
> On Tue, Oct 05, 2021 at 01:17:59PM +0200, Christian Schoenebeck wrote:
> > On Dienstag, 5. Oktober 2021 09:16:07 CEST Michael S. Tsirkin wrote:
> > > On Mon, Oct 04, 2021 at 09:38:08PM +0200, Christian Schoenebeck wrote:
> > > > Raise the maximum possible virtio transfer size to 128M
> > > > (more precisely: 32k * PAGE_SIZE). See previous commit for a
> > > > more detailed explanation for the reasons of this change.
> > > > 
> > > > For not breaking any virtio user, all virtio users transition
> > > > to using the new macro VIRTQUEUE_LEGACY_MAX_SIZE instead of
> > > > VIRTQUEUE_MAX_SIZE, so they are all still using the old value
> > > > of 1k with this commit.
> > > > 
> > > > On the long-term, each virtio user should subsequently either
> > > > switch from VIRTQUEUE_LEGACY_MAX_SIZE to VIRTQUEUE_MAX_SIZE
> > > > after checking that they support the new value of 32k, or
> > > > otherwise they should replace the VIRTQUEUE_LEGACY_MAX_SIZE
> > > > macro by an appropriate value supported by them.
> > > > 
> > > > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > > 
> > > I don't think we need this. Legacy isn't descriptive either.  Just leave
> > > VIRTQUEUE_MAX_SIZE alone, and come up with a new name for 32k.
> > 
> > Does this mean you disagree that on the long-term all virtio users should
> > transition either to the new upper limit of 32k max queue size or
> > introduce
> > their own limit at their end?
> 
> depends. if 9pfs is the only one unhappy, we can keep 4k as
> the default. it's sure a safe one.
> 
> > Independent of the name, and I would appreciate for suggestions for an
> > adequate macro name here, I still think this new limit should be placed in
> > the shared virtio.h file. Because this value is not something invented on
> > virtio user side. It rather reflects the theoretical upper limited
> > possible with the virtio protocol, which is and will be common for all
> > virtio users.
> We can add this to the linux uapi headers, sure.

Well, then I wait for few days, and if nobody else cares about this issue, 
then I just hard code 32k on 9pfs side exclusively in v3 for now and that's 
it.

Best regards,
Christian Schoenebeck