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Subject: Re: [virtio] Re: [virtio-comment] Re: [virtio] Re: [PATCH v10 04/10] admin: introduce virtio admin virtqueues
On Wed, Mar 08, 2023 at 01:57:43PM +0100, Jiri Pirko wrote: > Wed, Mar 08, 2023 at 01:44:18PM CET, stefanha@redhat.com wrote: > >On Wed, Mar 08, 2023 at 11:17:35AM +0100, Jiri Pirko wrote: > >> Tue, Mar 07, 2023 at 08:03:47PM CET, stefanha@redhat.com wrote: > >> >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. > >> > >> But isn't this duplicating the burden of maintaining dependencies to > >> both driver and device? I mean, device should not depend on driver doing > >> the right thing, that means it has to check the dependencies for every > >> incoming command anyway. The only difference would be to wait instead of > >> returning "-EBUSY" in case the dependency is not satisfied yet. > > > >The device does not need to reject commands that have dependencies with > >-EBUSY. The result of commands with dependencies is either A -> B or B > >-> A. > > > >For example: > >1. Create Group Member A > >2. Delete Group Member A > > > >Command 2 might succeed or it might fail with -ENOENT because Group > >Member A doesn't exist yet. > > Yeah, correct. > > > > >> Device knows exactly what are the dependencies. And I believe, those are > >> device implementation specific. For example, some implementation could > >> support parallel VF config cmd execution, some implementation might > >> need to serialize that. Driver has no clue. > > > >Yes, that's up to the device. Out-of-order completion is a superset of > >in-order completion. So the device is allowed to run commands in series > >when it wants. A driver designed for out-of-order completion will work > >fine either way. > > Agreed. > > > > > >> Could you please elaborate a bit more what you mean by "complexity in > >> the spec"? > > > >When adding commands to the spec, the dependency relationships with > >other commands need to be thought about and documented. > > > >Device implementers need to get those relationships right. That means > >they need to remember that command B waits for command A. > > That's my point, this is device implementation specific. Spec should not > define anything like this, that might be limiting in some of the cases. > > > > >Driver implementers have to understand that command B waits for command > >A but not command C according to the spec. > > > >That seems complex to me. > > Sure, I agree this does not belong to the spec, it can't, really. > For the record, I never suggested it. > > I think we are in agreement. Okay. In that case I think I misunderstood what you meant. Sorry! Stefan
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