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Date: Fri, 23 Feb 2018 18:05:25 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v8 09/21] null: Switch to
 .bdrv_co_block_status()
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To: Eric Blake <eblake@redhat.com>
Cc: qemu-devel@nongnu.org, famz@redhat.com, qemu-block@nongnu.org, Max Reitz <mreitz@redhat.com>

Am 23.02.2018 um 17:43 hat Eric Blake geschrieben:
> > OFFSET_VALID | DATA might be excusable because I can see that it's
> > convenient that a protocol driver refers to itself as *file instead of
> > returning NULL there and then the offset is valid (though it would be
> > pointless to actually follow the file pointer), but OFFSET_VALID without
> > DATA probably isn't.
> 
> So OFFSET_VALID | DATA for a protocol BDS is not just convenient, but
> necessary to avoid breaking qemu-img map output.  But you are also right
> that OFFSET_VALID without data makes little sense at a protocol layer. So
> with that in mind, I'm auditing all of the protocol layers to make sure
> OFFSET_VALID ends up as something sane.

That's one way to look at it.

The other way is that qemu-img map shouldn't ask the protocol layer for
its offset because it already knows the offset (it is what it passes as
a parameter to bdrv_co_block_status).

Anyway, it's probably not worth changing the interface, we should just
make sure that the return values of the individual drivers are
consistent.

Kevin