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

From: Fam Zheng
Subject: Re: [Qemu-devel] [PATCH v6 01/13] block: Add op blocker type "device IO"
Date: Thu, 28 May 2015 20:04:23 +0800
User-agent: Mutt/1.5.23 (2014-03-12)

On Thu, 05/28 13:47, Paolo Bonzini wrote:
> On 28/05/2015 13:44, Fam Zheng wrote:
> > > The reason for doing it in the block layer is that it's in one place and
> > > we can be sure that it's applied. We can still in addition modify
> > > specific users to avoid even trying to send requests, but I think it's
> > > good to have the single place that always ensures correct functionality
> > > of the drain instead of making it dependent on the user.
> > 
> > How to do that for the synchronous blk_write callers that don't run in
> > a coroutine?
> They would be completely oblivious to it.
> Their call to blk_co_write would queue the request.  Then blk_write
> calls aio_poll, which ultimately would result in blk_resume and unqueue
> the request.

OK.. I thought that, then we have to make a rule that a blk_pause must be
resolved by an aio_poll() loop.  This is somehow tricky, for example I assume
blk_write() would only call aio_poll() of its own AioContext? But the pairing
blk_resume() will be in the main context.  To make it worse, what if there is
no BDS in the main context, should it be special cased in the block layer?


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