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

[Sasha Levin <levinsasha928@gmail.com>]

On Wed, 2011-11-09 at 22:52 +0200, Michael S. Tsirkin wrote:
> 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?

Do you mean comments such as "For backwards compatibility, devices
should also present legacy configuration space in the first I/O region
of the PCI device"? What I understood from it is that the device should
have a legacy config in case it's used with an older guest, but I didn't
understand from it that the legacy config will be used even if new
layout is present.

> > It'll be a bit harder deprecating it in the future.
> 
> Harder than ... what ?

Harder than allowing devices not to present it at all if new layout
config is used. Right now the simple implementation is to use MMIO for
config and device specific, and let it fallback to legacy for ISR and
notifications (and therefore, this is probably how everybody will
implement it), which means that when you do want to deprecate legacy,
there will be extra work to be done then, instead of doing it now.

> IMO there's no way to put legacy anywhere except the first BAR
> without breaking existing guests.

It's not about where we put legacy, it's about how easy it is to drop
legacy entirely.

-- 

Sasha.