[virtio-comment] Re: VIRTIO_RING_F_INDIRECT_SIZE status

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 06, 2023 at 04:00:37PM +0100, Christian Schoenebeck wrote:
> On Wednesday, March 1, 2023 3:55:57 PM CET Michael S. Tsirkin wrote:
> > On Wed, Mar 01, 2023 at 01:55:14PM +0100, Christian Schoenebeck wrote:
> > >   2.8 Packed Virtqueues
> > >   ...
> > >   2.8.5 Scatter-Gather Support [1]
> > >   ...
> > >   While unusual (most implementations either create all lists solely using   
> > >   non-indirect descriptors, or always use a single indirect element), if both 
> > >   features have been negotiated, mixing indirect and non-indirect descriptors 
> > >   in a ring is valid, as long as each list only contains descriptors of a 
> > >   given type.
> > > 
> > >   [1] https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.html#x1-770005
> > > 
> > > To avoid misapprehensions: the way I understand it, same restrictions apply to
> > > packed queues as split queues, in the sense that you may neither chain several
> > > tables in a single message, nor multi-level nest tables, nor mix a list of
> > > direct descriptors and indirect descriptors on the same level within one
> > > message. So the explicit exception described here, only means you may use
> > > *one* indirect table in one message, while using chained direct descriptors in
> > > another message. But that's it, right?
> > 
> > 
> > That's my understanding.
> > 
> > > > 2. Given this is a lot of work I am trying to find a way to
> > > > make the impact bigger. In particular to cover the use-case
> > > > of limiting s/g to 1k while making queues deeper (with
> > > > or without indirect). For this I proposed:
> > > > 
> > > > 	So I think that given this, we can limit the total number
> > > > 	of non-indirect descriptors, including non-indirect ones
> > > > 	in a chain + all the ones in indirect pointer table if any,
> > > > 	and excluding the indirect descriptor itself, and this
> > > > 	will address the issue you are describing here, right?
> > > > 
> > > > people seemed to be ok with this idea?
> > > 
> > > IIUIC it would not make a difference from design perspective from what I
> > > proposed, as virtio currently neither allows to mix, chain or mult-level nest
> > > indirect descriptor tables within a single message), and hence it would just
> > > boil down to adjusting the wording. So yes, it would therefore cover my
> > > intended use case.
> > > 
> > > Best regards,
> > > Christian Schoenebeck
> > 
> > 
> > Sounds good to me. One interesting case is scsi and blk which have
> > a seg_max field. This is defined as
> > 
> > \item[\field{seg_max}] is the maximum number of segments that can be in a
> >     command. A bidirectional command can include \field{seg_max} input
> >     segments and \field{seg_max} output segments.
> > 
> > it is never explained what *are* the segments, or how does it
> > interact with VQ depth. Current drivers interpret this
> > strictly and assume that this limits the s/g length but does not
> > allow you to exceed vq size.
> > 
> > Do we thus want two limits (for read and write descriptors)?
> 
> No opinion on that, as my intended use case was just extending the buffer size
> beyond queue size, not limiting it below queue size. Either way is fine with
> me.
> 
> Anyhow, as this now gets broader scope, that also means the suggested flag
> VIRTIO_RING_F_INDIRECT_SIZE needs to be renamed. VIRTIO_RING_F_BUFFER_SIZE?
> 
> Best regards,
> Christian Schoenebeck


Hmm that's unclear in that it might be in bytes too.
Given blk and scsi call these "segments" how about
VIRTIO_RING_F_SEG_MAX?


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