linux-kernel - Re: [Xen-devel] [PATCH RFC] xen-block: correctly define structures in public headers

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Date:	Tue, 3 Dec 2013 11:15:22 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	Roger Pau Monne <roger.pau@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Julien Grall <julien.grall@...aro.org>,
	<xen-devel@...ts.xenproject.org>,
	BorisOstrovsky <boris.ostrovsky@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH RFC] xen-block: correctly define structures
 in public headers

On Tue, 2013-12-03 at 11:11 +0000, Jan Beulich wrote:
> >>> On 03.12.13 at 12:05, Ian Campbell <Ian.Campbell@...rix.com> wrote:
> > On Tue, 2013-12-03 at 11:57 +0100, Roger Pau Monne wrote:
> >> Using __packed__ on the public interface is not correct, this
> >> structures should be compiled using the native ABI, and __packed__
> >> should only be used in the backend counterpart of those structures
> >> (which needs to handle different ABIs).
> >> 
> >> This was even worse in the ARM case, where the Linux kernel was
> >> incorrectly using the X86_32 protocol ABI. This patch fixes it, but
> >> also breaks compatibility, so an ARM DomU kernel compiled with
> >> this patch will fail to communicate with PV disk devices unless the
> >> Dom0 also has this patch.
> > 
> > This is acceptable IMHO, the ARM ABI is clearly defined and previous
> > kernels were simply buggy. The fact that front and backend were
> > equivalently buggy and so it happened to work is not an excuse.
> 
> But afaics the change is not just to the ARM form of the ABI,
> but to x86 (32- and 64-bit) too. And that clearly must not be
> altered.

I understood there to be no (intentional) change to the x86 ABIs here.
Obviously that would be a bug in the patch.

Ian.

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