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

[Christian Schoenebeck]

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