linux-kernel - Re: [PATCHv2 RFC] virtio-spec: flexible configuration layout

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Message-ID: <20111109205208.GA28599@redhat.com>
Date:	Wed, 9 Nov 2011 22:52:08 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	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,
	avi@...hat.com, penberg@...nel.org
Subject: Re: [PATCHv2 RFC] virtio-spec: flexible configuration layout

On Wed, Nov 09, 2011 at 10:24:47PM +0200, Sasha Levin wrote:
> On Wed, 2011-11-09 at 21:59 +0200, Michael S. Tsirkin wrote:
> 
> [snip]
> 
> > +\begin_layout Enumerate
> > +Reset the device.
> > + This is not required on initial start up.
> > +\end_layout
> > +
> > +\begin_layout Enumerate
> > +The ACKNOWLEDGE status bit is set: we have noticed the device.
> > +\end_layout
> > +
> > +\begin_layout Enumerate
> > +The DRIVER status bit is set: we know how to drive the device.
> > +\end_layout
> > +
> > +\begin_layout Enumerate
> > +
> > +\change_inserted 1986246365 1320838089
> > +PCI capability list scan, detecting virtio configuration layout using Virtio
> > + Structure PCI capabilities.
> 
> Does the legacy space always gets mapped from BAR0?
> 
> If yes,

Yes and this is repeated in several places. Not clear? How can this
be made clearer?

> It'll be a bit harder deprecating it in the future.

Harder than ... what ?
IMO there's no way to put legacy anywhere except the first BAR
without breaking existing guests.

> If not, this should actually be #1. Previous 3 bullets here were already
> setting the status byte in the device config.
> 
> [snip]
> 
> > +<cell alignment="center" valignment="top" topline="true" leftline="true" usebox="none">
> > +\begin_inset Text
> > +
> > +\begin_layout Plain Layout
> > +
> > +\change_inserted 1986246365 1320786225
> > +28 bytes
> 
> This is the new virtio header min size, shouldn't it be 32? (20 + 4
> (msi-x) + 4 (guest feature) + 4 (device features)).

Good catch, I'll correct this.

> -- 
> 
> Sasha.
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