linux-kernel - Re: [PATCHv3 RFC] virtio-pci: flexible configuration layout

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Message-ID: <87vcq1rp0d.fsf@rustcorp.com.au>
Date:	Thu, 01 Dec 2011 13:12:42 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	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>,
	Pawel Moll <pawel.moll@....com>,
	Wang Sheng-Hui <shhuiw@...il.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: [PATCHv3 RFC] virtio-pci: flexible configuration layout

On Wed, 30 Nov 2011 15:12:43 +0200, Sasha Levin <levinsasha928@...il.com> wrote:
> On Wed, 2011-11-30 at 10:10 +1030, Rusty Russell wrote:
> > On Mon, 28 Nov 2011 11:15:31 +0200, Sasha Levin <levinsasha928@...il.com> wrote:
> > > On Mon, 2011-11-28 at 11:25 +1030, Rusty Russell wrote:
> > > > I'd like to see kvmtools remove support for legacy mode altogether,
> > > > but they probably have existing users.
> > > 
> > > While we can't simply remove it right away, instead of mixing our
> > > implementation for both legacy and new spec in the same code we can
> > > split the virtio-pci implementation into two:
> > > 
> > > 	- virtio/virtio-pci-legacy.c
> > > 	- virtio/virtio-pci.c
> > > 
> > > At that point we can #ifdef the entire virtio-pci-legacy.c for now and
> > > remove it at the same time legacy virtio-pci is removed from the kernel.
> > 
> > Hmm, that might be neat, but we can't tell the driver core to try
> > virtio-pci before virtio-pci-legacy, so we need detection code in both
> > modules (and add a "force" flag to virtio-pci-legacy to tell it to
> > accept the device even if it's not a legacy-only one).
> 
> I was thinking more in the direction of fallback code in virtio-pci.c to
> virtio-pci-legacy.c.
> 
> Something like:
> #ifdef VIRTIO_PCI_LEGACY
> [Create BAR0 and map it to virtio-pci-legacy.c]
> #endif
> 
> So BAR0 isn't defined as long as legacy code is there, which makes
> falling back to legacy pretty simple.

But it's nicer to see the driver actually labelled "virtio-pci-legacy",
and such a module.

I'll code something up, see what it looks like.

Cheers,
Rusty.
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