Re: VIRTIO_RING_F_INDIRECT_SIZE status

[Stefan Hajnoczi <stefanha@redhat.com>]

On Tue, Mar 07, 2023 at 11:47:32AM -0500, Michael S. Tsirkin wrote:
> On Tue, Mar 07, 2023 at 08:26:27AM -0500, Stefan Hajnoczi wrote:
> > On Mon, Mar 06, 2023 at 04:50:53PM -0500, Michael S. Tsirkin wrote:
> > > On Mon, Mar 06, 2023 at 03:46:01PM -0500, Stefan Hajnoczi wrote:
> > > > 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".
> > > 
> > > Hmm true:
> > > 	A buffer consists of zero or more device-readable physically-contiguous
> > > 	elements followed by zero or more physically-contiguous
> > > 	device-writable elements (each buffer has at least one element).
> > > 
> > > However we then need to clean this up, since
> > > 
> > > - At least in one place we say
> > > 
> > > indirect elements to mean indirect descriptors.
> > > 
> > > - we also say "queue elements" to mean "avail/desc/used"
> > > - We also say "descriptor elements" - not 100% sure it's the same.
> > > 
> > > so we need to clean this up a bit first and maybe add
> > > text about indirect descriptors not counting as elements.
> > 
> > Haha, yes. I also remembered that QEMU's type for a virtqueue buffer is
> > called VirtQueueElement :).
> > 
> > My impression from the spec is that when talking about virtqueues an
> > element is a data blob that's part of a buffer and when talking about
> > vrings an element descriptor is the ring entry that points to the data
> > blob. Often the terms are used interchangeably (just "descriptors" or
> > "elements").
> > 
> > I'm not sure if the distinction is necessary. It might be simpler to
> > always talk about descriptors and remove the term "element", since there
> > is no way to avoid talking about descriptors eventually.
> > 
> > Stefan
> 
> Well this whole discussion started from the point that
> indirect descriptors do not describe parts of a buffer.
> 
> So I feel there's a distinction.

Good point.

Stefan

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