linux-kernel - Re: [PATCH 1/4] virtio_blk: deprecate the 1024-byte ID field.

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Message-ID: <4ACA1CBC.8020402@redhat.com>
Date:	Mon, 05 Oct 2009 12:20:12 -0400
From:	john cooper <john.cooper@...hat.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>
CC:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, john.cooper@...hat.com
Subject: Re: [PATCH 1/4] virtio_blk: deprecate the 1024-byte ID field.

Christian Borntraeger wrote:
> Am Dienstag 29 September 2009 19:18:09 schrieb Rusty Russell:
>> PCI, lguest and s390 can all only support 256-byte configuration
>> space.  So, this giant field broke just about everyone.
>> Unfortunately, removing it is not so simple: we don't want to break
>> old userspace, but we're going to want to re-use that part of the
>> struct.
>>
>> So, modern users can #define VIRTIO_BLK_IDENTIFY_DEPRECATED to indicate
>> that they know it's no longer in the config struct, and can use any
>> new features (all new features which add a configuration field will
>> conflict with this deprecated one).
> 
> 
> Since s390 never used the giant id field, it would be ok for us just delete it 
> (without the #define). IIRC kvm-userspace also never used that. Since qemu 
> upstream seems to use that field your way seems to be the only compatible...

It seems simply retiring use of the VIRTIO_BLK_F_IDENTIFY
binary value should be sufficient.  The patch which
implemented the cooperating logic in qemu was dropped
due to the config space issue.  So the case of a virtio_blk
driver successfully negotiating the feature and trying to
access the now depreciated structure area should effectively
be a non-issue.  The bit value can be reclaimed for reuse
in the future when concern over any suspected usage falls
into the noise.

-john

-- 
john.cooper@...hat.com
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