[virtio-comment] Re: VIRTIO_RING_F_INDIRECT_SIZE status

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Mar 06, 2023 at 12:41:25PM -0500, Michael S. Tsirkin wrote:
> 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?

The VIRTIO equivalent of a "segment" is an "element". I don't think the
term "segment" is needed at the VIRTIO device model level since there is
already a word for it.

I'm confused because VIRTIO_RING_F_BUFFER_SIZE and VIRTIO_RING_F_SEG_MAX
mean different things to me and have different units (bytes vs number of
segments).

I wouldn't worry about virtio-blk/scsi seg_max. Although the segments
map to virtqueue elements, seg_max has a specific SCSI/block level
meaning related to data transfer and is not about constraints that apply
to all virtqueue requests. I/O requests have headers/footers, so they
can actually consume more elements than seg_max. Also, there could be
non-data transfer requests that happen to consume more than seg_max and
the storage controller would be happy with that (e.g. because VIRTIO
mandates flexible framing so you could break a request into 1-byte
elements). It's confusing the talk about seg_max at the VIRTIO device
model level - it's not about virtqueues at all.

Stefan

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