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

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Date:	Wed, 09 Nov 2011 14:48:58 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Avi Kivity <avi@...hat.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>,
	Pawel Moll <pawel.moll@....com>,
	Wang Sheng-Hui <shhuiw@...il.com>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: [PATCH RFC] virtio-spec: flexible configuration layout

On Wed, 2011-11-09 at 14:38 +0200, Avi Kivity wrote:
> On 11/09/2011 10:46 AM, Sasha Levin wrote:
> > > Alternatively we can add new structures with new
> > > structure IDs, pointed to from PCI configuration space.
> > > 
> > > As we don't yet have devices or drivers with 64 bit features,
> > > I decided we don't need high feature bits in legacy space.
> > > This also frees up feature bit 31 as we don't need it
> > > to enable high feature bits anymore.
> >
> > KVM tool actually has support for 64bit features, we can probably remove
> > that when Pekka isn't looking :)
> >
> 
> What about the Windows drivers?

While 64-bit features were defined, there was no actual devices to use
them.

So even if Windows drivers had the ability to support 64-bit features,
no device ever needed it therefore no device ever activated it.

A potential issue might arise when we remap feature bit 31 to do
something else, and when activated it would fool Windows drivers to
think that 64-bit features are now activated, while they're not - but
thats not something that should happen since legacy layout shouldn't
have that field enabled as per the spec (no new features in legacy
code).

I was unable to check if it was actually implemented in the drivers
because
http://git.kernel.org/?p=virt/kvm/kvm-guest-drivers-windows.git;a=summary is not quite there (*cough*).

-- 

Sasha.

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