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

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Message-ID: <20131203160018.67542a53@alan.etchedpixels.co.uk>
Date:	Tue, 3 Dec 2013 16:00:18 +0000
From:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
To:	Julien Grall <julien.grall@...aro.org>
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 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 ?

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

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.

Alan
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