linux-kernel - Re: [PATCH v2] xen/block: Correctly define structures in public headers on ARM32 and ARM64

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Date:	Tue, 03 Dec 2013 16:19:55 +0000
From:	Julien Grall <julien.grall@...aro.org>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
CC:	xen-devel@...ts.xenproject.org, patches@...aro.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Roger Pau Monne <roger.pau@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>
Subject: Re: [PATCH v2] xen/block: Correctly define structures in public headers
 on ARM32 and ARM64

On 12/03/2013 04:00 PM, One Thousand Gnomes wrote:
> On Tue,  3 Dec 2013 15:40:37 +0000
> Julien Grall <julien.grall@...aro.org> wrote:
> 
>> On ARM (32 bits and 64 bits), the double-word is 8-bytes aligned. This will
>> result on different structure from Xen and Linux repositories.
>>
>> As Linux is using __packed__ attribute, it must have a 4-bytes padding before
>> each "id" field.
>>
>> This change breaks guest block support with older kernel. IMHO, it's acceptable
>> because Xen on ARM is still on Tech Preview and the hypercall ABI is not yet
>> freezed.
> 
> How does a guest ascertain which API to use ?
> 
> How does the patch ensure new kernels on existing hypervisor versions
> don't break ?

As Ian said on the thread "xen-block: correctly define structures in
public headers" (see thread https://lkml.org/lkml/2013/12/3/155), the
ABI is not yet fixed for ARM.

> 
> What is the failure case given the alignment change seems potentially to
> produce valid but incorrect I/O requests - can it cause corruption ?

The request ID will likely be wrong, so the guest won't accept the
request. It should not corrupt the block device.

> It seems to me you should be defining
> 
> struct blkif_request_rw_v2
> 
> and using the correct version according to which API the hypervisor
> requires, not just breaking it.

This API doesn't involve the hypervisor. It's only a way to talk between
DOM0 and a guest. Without this change you will break compatibility with
other OSes.

-- 
Julien Grall
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