Re: [PATCH] context: Document VIRTIO_F_IOMMU_PLATFORM reserved feature bit (33)

[Will Deacon <will.deacon@arm.com>]

On Thu, Oct 20, 2016 at 07:54:30PM +0300, Michael S. Tsirkin wrote:
> On Tue, Oct 11, 2016 at 01:51:32PM +0100, Will Deacon wrote:
> > diff --git a/content.tex b/content.tex
> > index 222b78e7f5d8..1240d0005d77 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -95,10 +95,10 @@ Feature bits are allocated as follows:
> >  \begin{description}
> >  \item[0 to 23] Feature bits for the specific device type
> >  
> > -\item[24 to 32] Feature bits reserved for extensions to the queue and
> > +\item[24 to 33] Feature bits reserved for extensions to the queue and
> >    feature negotiation mechanisms
> >  
> > -\item[33 and above] Feature bits reserved for future extensions.
> > +\item[34 and above] Feature bits reserved for future extensions.
> >  \end{description}
> >  
> >  \begin{note}
> > @@ -5813,7 +5813,7 @@ descriptor for the \field{sense_len}, \field{residual},
> >  
> >  \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
> >  
> > -Currently there are three device-independent feature bits defined:
> > +Currently there are four device-independent feature bits defined:
> >  
> >  \begin{description}
> >    \item[VIRTIO_F_RING_INDIRECT_DESC (28)] Negotiating this feature indicates
> > @@ -5825,6 +5825,12 @@ Currently there are three device-independent feature bits defined:
> >  
> >    \item[VIRTIO_F_VERSION_1(32)] This indicates compliance with this
> >      specification, giving a simple way to detect legacy devices or drivers.
> > +
> > +  \item[VIRTIO_F_IOMMU_PLATFORM(33)] This feature indicates that the device is
> > +  behind an IOMMU that translates bus addresses from the device into physical
> > +  addresses in memory.  If this feature bit is set to 0, then the device emits
> > +  physical addresses which are not translated further, even though an IOMMU
> > +  may be present.
> >  \end{description}
> >  
> >  \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
> > @@ -5832,11 +5838,21 @@ Currently there are three device-independent feature bits defined:
> >  A driver MUST accept VIRTIO_F_VERSION_1 if it is offered.  A driver
> >  MAY fail to operate further if VIRTIO_F_VERSION_1 is not offered.
> >  
> > +A driver MUST accept VIRTIO_F_IOMMU_PLATFORM if it is offered,
> 
> I would make it a SHOULD, we can't really add requirements
> that make existing confirmant drivers non-confirmant.
> 
> > and it MUST then
> 
> .. either disable the IOMMU or ..

Okey doke, with that qualification then this sounds good to me. I'll spin
a v2 with these changes and the other suggestion from Stefan.

Cheers for the review,

Will