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Date: Tue, 01 Jul 2014 18:56:13 +0200
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Subject: Re: [Qemu-devel] [PATCH v2 1/3] block: block: introduce
	bdrv_io_plug() and bdrv_io_unplug()
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To: Kevin Wolf <kwolf@redhat.com>, Ming Lei <ming.lei@canonical.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>

Il 01/07/2014 17:21, Kevin Wolf ha scritto:
>>>> Does this bs->file forwarding work for more than the raw driver? For
>>>> example, if drv is an image format driver that needs to read some
>>>> metadata from the image before it can submit the payload, does this
>>>> still do what you were intending?
>>
>> Sorry for not understanding the problem, and you are right, these
>> patches can't support other formats, and for solving the dependency,
>> changes to image format driver should be needed.
>
> Then let's drop the bs->file recursion here and add an explicit
> .bdrv_io_plug/unplug callback to the raw driver.

Actually I thought about this in my review, and there's no reason for 
this not to work for image formats.

While bs->file is plugged, image formats will start executing their 
bdrv_co_readv/bdrv_co_writev callbacks, and issue reads or writes as 
necessary.  The reads and writes will accumulate in bs->file until it is 
unplugged, which is exactly the effect we want.

The change in bdrv_drain_all is ugly though.  I don't have a better 
idea, but I would like to understand better why it is needed.  Ming Lei, 
did you see a deadlock without it?

Paolo