[virtio-comment] Re: [virtio-dev] Re: [PATCH v5 2/5] pci: Define id field

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Cornelia Huck (cohuck@redhat.com) wrote:
> On Thu, 27 Jun 2019 20:28:27 +0100
> "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:
> 
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > For the virtio-fs device we require multiple large shared memory
> > regions.  Differentiate these by an 'id' field in the base capability.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  content.tex | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/content.tex b/content.tex
> > index 6433226..41926c0 100644
> > --- a/content.tex
> > +++ b/content.tex
> > @@ -651,7 +651,8 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
> >          u8 cap_len;     /* Generic PCI field: capability length */
> >          u8 cfg_type;    /* Identifies the structure. */
> >          u8 bar;         /* Where to find it. */
> > -        u8 padding[3];  /* Pad to full dword. */
> > +        u8 id;          /* Multiple capabilities of the same type */
> > +        u8 padding[2];  /* Pad to full dword. */
> >          le32 offset;    /* Offset within bar. */
> >          le32 length;    /* Length of the structure, in bytes. */
> >  };
> > @@ -716,6 +717,11 @@ \subsection{Virtio Structure PCI Capabilities}\label{sec:Virtio Transport Option
> >  
> >          Any other value is reserved for future use.
> >  
> > +\item[\field{id}]
> > +        Multiple capabilities of the same type can exist as long
> > +        as they each have a unique \field{id}.  The specific
> 
> The requirement for id is new, isn't it? Shouldn't it rather be
> optional?

Yes it is; this comes from moving it from the new structure we had it in
the previous version, into the main _cap.

Can I just say that only some devices use it, or do I need something
more?   Can I assume that 'id' is 0 for existing devices or is it
undefined?

> > +        meaning of the field is different for each device type.
> > +
> >  \item[\field{offset}]
> >          indicates where the structure begins relative to the base address associated
> >          with the BAR.  The alignment requirements of \field{offset} are indicated
> 
> The current specification for cfg_type defines a kind of hierarchy for
> capabilities of the same type (first one is preferred). We probably
> need to be more explicit how id may interact with that (even if it is
> device type specific).

One way I was thinking was to say that the hierarchy now applies to
pairs of {cfg_type, id} rather than just cfg_type; but that depends on
the answer to the previous question to avoid breaking any existing
hierarchies.

Dave



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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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