linux-kernel - Re: [PATCH RFC] virtio-spec: flexible configuration layout

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Message-ID: <1320835653.3259.138.camel@hornet.cambridge.arm.com>
Date:	Wed, 09 Nov 2011 10:47:33 +0000
From:	Pawel Moll <pawel.moll@....com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alexey Kardashevskiy <aik@...abs.ru>,
	Amit Shah <amit.shah@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Krishna Kumar <krkumar2@...ibm.com>,
	Wang Sheng-Hui <shhuiw@...il.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"avi@...hat.com" <avi@...hat.com>
Subject: Re: [PATCH RFC] virtio-spec: flexible configuration layout

On Wed, 2011-11-09 at 10:20 +0000, Sasha Levin wrote:
> > > I'm also wondering it it's ok to move virtio configuration out of virtio
> > > space and into PCI space for archs that don't have PCI (such as ARM).

Just a note - ARM-based chips can by all means have PCI (grep -r PCI
arch/arm/ ;-). The fact is that most of the SOCs available on the market
don't have it, but this is slowly changing.

The main architectural difference is that ARM doesn't provide separate
I/O space so the PCI I/O space is usually remapped somewhere into normal
address space (grep -r "#define __io_address" arch/arm/)
 
> > > Would it mean they get stuck with legacy configuration (and no new
> > > features)? Or is there an alternative for them?
> > 
> > The change only affects the layout of virtio PCI. Arches that don't
> > have PCI don't use virtio PCI, presumably?
> > 
> > BTW, the spec only covers x86 ATM, this needs to be fixed.
> 
> From what I see there is a WIP by Pawel Moll <pawel.moll@....com> to add
> virtio platform drivers which get virtio working on ARM for example, and
> by Peter Maydell <peter.maydell@...aro.org> to modify the spec to
> support MMIO access (besides PCI).

Yep, it's actually already in 3.2-rc1 (drivers/virtio/virtio_mmio.c) and
in the spec (see Appendix X). And actually the control registers layout
I used was originally based on the PCI "legacy" headers (slightly
simplified), but evolved a bit since. My understanding is that the
changes Michael is proposing affect the PCI device interface only so
they shouldn't affect "my" interface.

By the way, I vaguely remember Peter mentioning that he got the PCI
device "experimentally" running some time ago on one of the PCI-enabled
ARM platform models (realview or versatile)...

Cheers!

Pawel



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