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Subject: Re: VIRTIO_RING_F_INDIRECT_SIZE status
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. -- MST
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