Re: [virtio-comment] Re: [PATCH v3 4/4] Add CCW configuration field "indirect_num"

[Halil Pasic <pasic@linux.ibm.com>]

On Fri, 18 Mar 2022 12:02:31 +0100
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Donnerstag, 17. MÃrz 2022 15:12:42 CET Cornelia Huck wrote:
> > On Wed, Mar 16 2022, Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:  
> > > This new CCW configuration field allows to negotiate a more fine
> > > graded maximum lenght of indirect descriptor chains.
> > > 
> > > Bump CCW virtio revision to 3 and make the existence of this new
> > > field "indirect_num" dependant on revision 3.
> > > 
> > > Fixes: https://github.com/oasis-tcs/virtio-spec/issues/122
> > > Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> > > ---
> > > 
> > >  content.tex | 14 +++++++++++++-
> > >  1 file changed, 13 insertions(+), 1 deletion(-)  
> > 
> > (...)
> >   
> > > @@ -2581,12 +2583,17 @@ \subsubsection{Configuring a
> > > Virtqueue}\label{sec:Virtio Transport Options / Vir> 
> > >  struct vq_config_block {
> > >  
> > >          be16 index;
> > >          be16 max_num;
> > > 
> > > +        be32 indirect_num; /* since virtio-ccw rev. 3 */  
> > 
> > Maybe make it max_indirect_num (to mirror max_num?)  
> 
> Right, that name appears indeed more appropriate here.
> 
> > >  };
> > >  \end{lstlisting}
> > >  
> > >  The requested number of buffers for queue \field{index} is returned in
> > >  \field{max_num}.
> > > 
> > > +Since revision 3, \field{indirect_num} exists, which is supposed to
> > > reflect the +Queue Indirect Size (i.e. the maximum length of indirect
> > > descriptor tables) +supported by device for this queue.  
> > 
> > It still depends on the feature flag, though.
> > 
> > Maybe
> > 
> > "If revision 2 or lower is set, struct vq_config_block extends up to
> > \field{max_num}.
> > 
> > If revision 3 or higher is set, struct vq_config_block extends to
> > \field{max_indirect_num}. If VIRTIO_RING_F_INDIRECT_SIZE is negotiated,
> > \field{indirect_max_num} contains the maximum Queue Indirect Size
> > (i.e. the maximum length of indirect descriptor tables) supported by the
> > device for this queue; otherwise, its contents are unpredictable."  
> 
> Maybe rather something like this instead:
> 
> "Actual size of struct vq_config_block depends on the virtio-ccw revision.
> 
> If revision 2 or lower is set, struct vq_config_block extends up to
> *including* \field{max_num}.
> 
> If revision 3 or higher is set, struct vq_config_block extends up to
> *including* \field{max_indirect_num}. If VIRTIO_RING_F_INDIRECT_SIZE is 
> negotiated, \field{indirect_max_num} contains the maximum Queue Indirect Size
> (i.e. the maximum length of indirect descriptor tables) supported by the
> device for this queue; otherwise, its contents are *undefined*."
>  

I agree that the "including" is important, but I'm not sure about the
"its contents are undefined". I don't really understand why should we use
a plural here. What speaks against specifying that in SHOULD be stored
as 0 by the device, and MUST be ignored by the driver?

Currently we say that \field{max_indirect_num} exists like a be32 field
even if VIRTIO_RING_F_INDIRECT_SIZE is not negotiated. Which kind of
implies that at least type invariants should hold. Of course, there is
none here (i.e. every bits value is also a be32 value), but for something
like an enum interesting corner cases can pop up.

[..]

Regards,
Halil