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

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Message-ID: <1322658763.3985.4.camel@lappy>
Date:	Wed, 30 Nov 2011 15:12:43 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
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, 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.

-- 

Sasha.

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