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Subject: Re: [virtio-comment] Re: [virtio] Re: [PATCH v10 04/10] admin: introduce virtio admin virtqueues
On Tue, Mar 07, 2023 at 04:07:54PM +0100, Jiri Pirko wrote: > Tue, Mar 07, 2023 at 03:39:11PM CET, stefanha@redhat.com wrote: > >On Tue, Mar 07, 2023 at 09:03:18AM +0100, Jiri Pirko wrote: > >> Mon, Mar 06, 2023 at 07:37:31PM CET, mst@redhat.com 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. > >> > >> Hmm, is it suitable to have this approach for configuration interface? > >> Storage device is a different beast, having parallel reads and writes > >> makes complete sense for performance. > >> > >> ->read a req > >> ->read b req > >> ->read c req > >> <-read a rep > >> <-read b rep > >> <-read c rep > >> > >> There is no dependency, even between writes. > >> > >> But in case of configuration, does not make any sense to me. > >> Why is it needed? To pass the burden of consistency of > >> configuration to driver sounds odd at least. > >> > >> I sense there is no concete idea about what the "admin virtqueue" should > >> serve for exactly. > > > >It's useful for long-running commands because they prevent other > >commands from executing. > > > >An example I've given is that deleting a group member might require > >waiting for the group member's I/O activity to finish. If that I/O > >activity cannot be cancelled instantaneously, then it could take an > >unbounded amount of time to delete the group member. The device would be > >unable to process futher admin commands. > > I see. Then I believe that the device should handle the dependencies. > Example 1: > -> REQ cmd to create group member A > -> REQ cmd to create group member B > <- REP cmd to create group member A > <- REP cmd to create group member B > > The device according to internal implementation can either serialize the > 2 group member creations or do it in parallel, if it supports it. > > Example 2: > -> REQ cmd to create group member A > -> REQ cmd config group member A > <- REP cmd to create group member A > <- REP cmd config group member A > > Here the serialization is necessary and the device is the one to take > care of it. > > Makes sense? Yes, I understand. The spec would need to define ordering rules for specific commands and the device must implement them. It allows the driver to pipeline commands while also allowing out-of-order completion (parallelism) in some cases. The disadvantage of this approach is complexity in the spec and implementations. An alternative is unconditional out-of-order completion, where there are no per-command ordering rules. The driver must wait for a command to complete if it relies on the results of that command for its next command. I like this approach because it's less complex in the spec and for device implementers, while the burden on the driver implementer is still reasonable. > > > >Group member creation might have similar issues if it involves acquiring > >remote resources (e.g. connecting to a Ceph cluster or allocating ports > >on a distributed network switch). It can be impossible to defer resource > > Sidetrack: this is really fuzzy to me, how the new member is going to be > plugged into backend (network). Over the time, we learned that the > creation of device from the other side (switch side) makes more sense. > That is why I asked for motivation to introduce this infra. Michael, have you already thought about this? > >acquisition/initialization because because VIRTIO devices must be > >available as soon as the driver can see them (i.e. how do populate > >Configuration Space fields if you don't have the details of the resource > >yet?). > > > >So I have raised two questions: > > > >1. What are the admin queue command completion semantics: in-order or > > out-of-order command completion? > > I would add "dependencies/serialization" here. > > > > > >2. Will there be long-running commands and how will we deal with them > > when they hang? > > Yeah, sounds legit to define it in spec. > > > > >Stefan > >
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