Re: [Qemu-devel] [PATCH][RFC] Linux AIO support when using O_DIRECT

From: Avi Kivity <avi@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: qemu-devel@nongnu.org, Christoph Hellwig <hch@infradead.org>,
	kvm@vger.kernel.org
Subject: Re: [Qemu-devel] [PATCH][RFC] Linux AIO support when using O_DIRECT
Date: Mon, 23 Mar 2009 21:35:37 +0200	[thread overview]
Message-ID: <49C7E489.4070604@redhat.com> (raw)
In-Reply-To: <20090323184827.GA14826@lst.de>

Christoph Hellwig wrote:
> On Mon, Mar 23, 2009 at 01:10:30PM -0500, Anthony Liguori wrote:
>   
>> I really dislike having so many APIs.  I'd rather have an aio API that 
>> took byte accesses or have pread/pwrite always be emulated with a full 
>> sector read/write
>>     
>
> I had patches to change the aio API to byte based access, and get rid
> of the read/write methods to only have the byte based pread/pwrite
> APIs, but thay got obsoleted by Avi's patch to kill the pread/pwrite
> ops.  We could put in byte-based AIO without byte-based read/write,
> though.  In my patches I put a flag into BlockDriverState whether we
> allow byte-based access to this instance or otherwise emulated it in
> the block layer.  

I like this approach.  An additional flag could tell us what buffer 
alignment the format driver wants, so we can eliminate the alignment 
bounce from format driver code.  Oh, and a flag to indicate we don't 
support vectors, so the generic layer will bounce and send us a length 
one iovec.

Note the align flag is in the device state, not the format driver, as it 
depends on the cache= settings.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.