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

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Date:	Tue, 3 Dec 2013 12:09:57 +0100
From:	Roger Pau Monné <roger.pau@...rix.com>
To:	David Vrabel <david.vrabel@...rix.com>
CC:	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
	Julien Grall <julien.grall@...aro.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>
Subject: Re: [PATCH RFC] xen-block: correctly define structures in public
 headers

On 03/12/13 12:01, David Vrabel wrote:
> On 03/12/13 10:57, 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 ABI change needs to be justified.  Why do you think it is
> acceptable to break existing Linux guests?  Because I don't think it is.

ARM Linux guests are not using the ABI found in Xen public headers, they
are using the X86_32 ABI, because of the __packed__ and #ifdef
CONFIG_X86_64 (which made ARM kernels use the X86_32 ABI).

If someone picks a pristine version of the structures found in blkif.h
from upstream Xen and compile them on ARM they will see that it differs
from what the Linux kernel currently generates, which is not acceptable.

IMHO keeping it like this is a big mistake, this prevents compatibility
with ARM guests that implement the correct ABI.

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