Re: [Qemu-devel] [patch v4 05/16] memory: introduce ref, unref interface for MemoryRegionOps

[Avi Kivity <avi@redhat.com>]

On 10/24/2012 09:29 AM, Paolo Bonzini wrote:
> Il 23/10/2012 18:09, Avi Kivity ha scritto:
>>> But our interfaces had better support asynchronicity, and indeed they
>>> do: after you write to the "eject" register, the "up" will show the
>>> device as present until after destroy is done.  This can be changed to
>>> show the device as present only until after step 4 is done.
>> 
>> Let's say we want to eject the hotplug hardware itself (just as an
>> example).  With refcounts, the callback that updates "up" will hold on
>> to to it via refcounts.  With stop_machine(), you need to cancel that
>> callback, or wait for it somehow, or it can arrive after the
>> stop_machine() and bite you.
> 
> The callback that updates "up" is for the parent of the hotplug
> hardware.  There is nothing that has to be updated in the hotplug
> hardware itself.

I meant, as an unrealistic example, hot-unplugging the bridge itself.
So we have a callback that updates information in the bridge (up
register state) being called asynchronously.

A more realistic example would be hot-unplug of an HBA, then the block
layer callback comes back to update the device.  So stop_machine() would
need to cancel all I/O and wait for I/O that cannot be cancelled.

> 
> Updating the "up" register is the final part of isolate(), and runs
> before the stop_machine().  The steps above can be further refined like
> this:
> 
> 4a. close all backends (also cancel or complete all pending I/O)

^ long latency

> 4b. notify parent that we're done
>     4ba. parent removes device from its bus
>     4bb. parent notifies guest
>     4bc. parent schedules stop_machine(qdev_free(child))
> 5. a bottom half calls stop_machine(qdev_free(child))
> 
> If unplugging a whole sub-tree, the parent can notify its own parent at
> the end of 4b.  Because the only purpose of stop_machine is to quiesce
> subsystems not affected by step 4 (timer+memory, typically),
> destructions can be done in any order and even intermixed with
> executions of 4b for the parent.
> 
> In the beginning the only asynchronous step would be 5.  If the need
> arises we can use continuation-passing to make all the preceding steps
> asynchronous too.
> 

Maybe my worry about long stop_machine latencies is premature.  Everyone
in the kernel hates it, but the kernel scales a lot more than qemu and
is in a much better place wrt threading.



-- 
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