Re: [Qemu-devel] [PATCH v6 01/13] block: Add op blocker type "device IO"

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/05/2015 11:07, Kevin Wolf wrote:
> This is the first part of what's troubling me with this series, as it
> makes me doubtful if op blockers are the right tool to implement what
> the commit message says (block device I/O). This is "are we doing the
> thing right?"
> 
> The second part should actually come first, though: "Are we doing the
> right thing?" I'm also doubtful whether blocking device I/O is even what
> we should do.
> 
> Why is device I/O special compared to block job I/O or NBD server I/O?

Because block job I/O doesn't modify the source disk.  For the target
disk, block jobs should definitely treat themselves as device I/O and
register notifiers that pause themselves on bdrv_drain.

> If the answer is "because block jobs are already paused while draining"
> (and probably nobody thought much about the NBD server), then chances
> are that it's not the right thing.  In fact, using two different
> mechanisms for pausing block jobs and pausing device I/O seems
> inconsistent and wrong.
> 
> I suspect that the real solution needs to be in the block layer, though
> I'm not quite sure yet what it would be like. Perhaps a function pair
> like blk_stop/resume_io() that is used by bdrv_drain() callers and works
> on the BlockBackend level.

This is suspiciously similar to the first idea that I and Stefan had,
which was a blk_pause/blk_resume API, implemented through a notifier list.

Paolo