linux-kernel - Re: [PATCH] virtio-spec: Define virtio-mmio registers as LE

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Message-ID: <51308F22.7040303@arm.com>
Date:	Fri, 01 Mar 2013 11:21:06 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Pawel Moll <Pawel.Moll@....com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH] virtio-spec: Define virtio-mmio registers as LE

On 01/03/13 10:50, Pawel Moll wrote:
> On Fri, 2013-03-01 at 10:41 +0000, Marc Zyngier wrote:
>> On 14/02/13 10:54, Pawel Moll wrote:
>>> To solve the never-ending confusions between hosts and guests
>>> of different endianess, define all virtio-mmio registers as LE.
>>>
>>> This change should be safe at this stage, because no known
>>> working mixed-endian system exists so there is virtually no
>>> risk of breaking compatibility.
>>>
>>> Signed-off-by: Pawel Moll <pawel.moll@....com>
>>
>> Shouldn't we exclude the config space? PCI defines it as guest-endian,
>> and the above tends to indicate that it should be LE with MMIO.
> 
> The spec says: "Device-specific configuration space starts at an offset
> 0x100 and is accessed with byte alignment. Its meaning and size depends
> on the device and the driver."
> 
> I would hope that "accessed with byte alignment" is enough of a clue in
> this subject, but if you think otherwise I can add a sentence stating
> this explicitly.

Well, it was unclear enough for me to get confused... ;-) It would make
sense to have a wording similar to the one in the PCI section.

> Having said that, Rusty was contemplating enforcing LE config space in
> the new PCI layout...

I wouldn't complain about that, and would like to see a similar thing on
MMIO.

	M.
-- 
Jazz is not dead. It just smells funny...

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