Bug 1211689

The idea is to add a bdrv_pause/bdrv_resume pair, where:

- bdrv_pause checks if there is an operation blocker for PAUSE.  if it is there, it fails
- otherwise, bdrv_pause invokes a notifier list if this is the outermost call. otherwise it does nothing
- bdrv_resume does the same, but does not need a blocker
- dataplane registers pause/resume notifiers for its BlockBackend, which respectively disconnect/reconnect the ioeventfd.
- callers of bdrv_drain/bdrv_drain_all are a good place where to add pause/resume.  Because bdrv_pause_all() makes little sense (e.g. if doing something related to the CPU, we do not care about pausing block job targets), bdrv_drain_all() in many cases should be replaced by bdrv_drain().
- for transactions, bdrv_drain_all() probably should be replaced by bdrv_drain() invocations in the prepare callback, so that the callback can also invoke bdrv_pause() before bdrv_drain().

Optionally:
- block jobs can register pause/resume notifiers for their targets, which pause/resume the job, or they can block PAUSE on their targets.  In particular, drive-backup should block PAUSE on its target, everything else can register notifiers.

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IRC conversation:

<bonzini> stefanha: iirc we never implemented "stop ioeventfd while the main context is acquiring an additional aiocontext", have we?
<stefanha> bonzini: The necessary AioContexts need to be acquired for the entire duration of the transaction,
<bonzini> stefanha: yes, but the VM can still write in the meanwhile
<stefanha> bonzini: I think AioContext acquire/release is oversimplifying the problem,
<stefanha> there are actually multiple mechanisms we need.
<stefanha> bonzini: In this case I think a device callback might make sense,
<bonzini> stefanha: yes, like a pause/restart notifier on AioContext
<stefanha> so the monitor can tell the device associated with a BlockBackend to stop submitting requests.
<bonzini> stefanha: exactly, pausing between acquire and bdrv_drain, and restarting before release
<stefanha> Yes
<stefanha> bonzini: there are two different concepts here:
<stefanha> 1. AioContext acquire/release is about thread-safety
<stefanha> 2. Device pause/resume is about quiescing I/O from the guest
<stefanha> I still think #1 is useful
<bonzini> stefanha: yes it definitely is. the important point for me is that thread-safety can be made as fine-grained as you want
<bonzini> stefanha: but pause/resume cannot be made fine-grained
<bonzini> stefanha: so far we had coarse-grained critical sections, so we could coalesce the two
<bonzini> stefanha: (could, but didn't since we never implemented #2 :D)
<stefanha> bonzini: We need to be careful about pause/resume
<stefanha> There are also timers and blockjobs
<stefanha> both could cause additional I/O
<stefanha> So even if we stop the device from submitting requests, we also need to pause blockjobs and maybe also timers.
<stefanha> We should do the same for QMP 'transaction'.
<bonzini> stefanha: it's in qemu-devel, i don't recall you merged it
<bonzini> stefanha: timers should be included in bdrv_drain_all, or disabled by other notifiers
<bonzini> stefanha: block jobs are interesting
<stefanha> e.g. drive-backup
<stefanha> It hooks into the write callback
<stefanha> The write request is paused until drive-backup has stashed away the data,
<stefanha> but we might hit a deadlock there if the blockjob is paused...
<stefanha> This is hard
* stefanha goes to write fizzbuzz instead
<bonzini> stefanha: perhaps pause/resume should have blockers too
<bonzini> stefanha: drive-backup can block pause/resume on the destination
<stefanha> bonzini: Yes, we can use op blockers
<bonzini> stefanha: and transaction can limit the pause/resume to things it has to operate on
<bonzini> stefanha: but streaming, for example, is compatible with pause/resume, because its writes do not change disk contents
<stefanha> So it's more nuanced than just 'writes are forbidden'