Re: [virtio] Re: [PATCH v10 04/10] admin: introduce virtio admin virtqueues

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

On Mon, Mar 06, 2023 at 03:17:59PM -0500, Stefan Hajnoczi wrote:
> On Mon, Mar 06, 2023 at 01:37:31PM -0500, Michael S. Tsirkin wrote:
> > On Mon, Mar 06, 2023 at 06:03:40AM -0500, Stefan Hajnoczi wrote:
> > > On Sun, Mar 05, 2023 at 07:18:24PM -0500, Michael S. Tsirkin wrote:
> > > > On Sun, Mar 05, 2023 at 07:03:02PM -0500, Stefan Hajnoczi wrote:
> > > > > On Sun, Mar 05, 2023 at 04:38:59AM -0500, Michael S. Tsirkin wrote:
> > > > > > On Fri, Mar 03, 2023 at 03:21:33PM -0500, Stefan Hajnoczi wrote:
> > > > > > > What happens if a command takes 1 second to complete, is the device
> > > > > > > allowed to process the next command from the virtqueue during this time,
> > > > > > > possibly completing it before the first command?
> > > > > > > 
> > > > > > > This requires additional clarification in the spec because "they are
> > > > > > > processed by the device in the order in which they are queued" does not
> > > > > > > explain whether commands block the virtqueue (in order completion) or
> > > > > > > not (out of order completion).
> > > > > > 
> > > > > > Oh I begin to see. Hmm how does e.g. virtio scsi handle this?
> > > > > 
> > > > > virtio-scsi, virtio-blk, and NVMe requests may complete out of order.
> > > > > Several may be processed by the device at the same time.
> > > > 
> > > > Let's say I submit a write followed by read - is read
> > > > guaranteed to return an up to date info?
> > > 
> > > In general, no. The driver must wait for the write completion before
> > > submitting the read if it wants consistency.
> > > 
> > > Stefan
> > 
> > I see.  I think it's a good design to follow then.
> > 
> > I'll just copy
> > 	The driver queues requests to an arbitrary request queue, and
> > 	they are used by the device on that same queue. It is the
> > 	responsibility of the driver to ensure strict request ordering
> > 	for commands placed on different queues, because they will be
> > 	consumed with no order constraints.
> > 
> > replacing "request" with "admin".
> 
> That sounds like it's only about multi-queue because it says "... for
> commands placed on different queues". What I mentioned about a write
> followed by a read quest also applies within a single queue.
> 
> Can you clarify the semantics in the single queue case?
> 
> Stefan

This will be addressed in the latest version.
However, should not scsi and blk also adjust the wording
to make it clear device does not guarantee consistency?

-- 
MST