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

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Date:	Wed, 06 Mar 2013 15:10:15 +0000
From:	Pawel Moll <pawel.moll@....com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Marc Zyngier <Marc.Zyngier@....com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH] virtio-spec: Define virtio-mmio registers as LE

On Tue, 2013-03-05 at 00:11 +0000, Rusty Russell wrote:
> Pawel Moll <pawel.moll@....com> writes:
> > On Fri, 2013-03-01 at 11:21 +0000, Marc Zyngier wrote:
> >> > 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.
> >
> > Wherever PCI goes, MMIO follows :-)
> 
> Yes, but if you switch from 'guest-endian' to 'little-endian' how will
> you tell?  For PCI, we'd detect it by using the new layout.

The version register/value. At some point of time there will be a
new(ish) MMIO layout anyway to deal with 64-bit addresses, replacing the
ring page number with two 32-bit hi/lo physical address registers. This
was discussed not long after the driver got merged...

> I'd rather you specify MMIO as little endian, and we fix the kernel
> config accessors to be endian aware (ie. 8, 16, 32, 64-bit accessors).
> Since noone BE is using MMIO right now, it's safe...

That's absolutely fine with me, however I don't see anything I could do
in the virtio_mmio driver and spec - the virtio_config_ops specifies
get/set as void * operations and I simply do byte-by-byte copy. Have I
missed some config/endianess/PCI related discussion?

Paweł


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