[virtio] Re: [PATCH] introduction: document bitfield notation

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 5 Mar 2018 16:26:11 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Mar 05, 2018 at 03:20:35PM +0100, Cornelia Huck wrote:
> > On Wed, 28 Feb 2018 21:16:32 +0200
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >   
> > > Bitfields are a useful and familiar way to specify sub-byte structure
> > > layout. The only issue is that bitfield order isn't portable across
> > > architectures.  Document that we list bitfields from least to
> > > most significant one, and warn about portability issues.
> > > 
> > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > ---
> > >  introduction.tex | 18 ++++++++++++++++++
> > >  1 file changed, 18 insertions(+)
> > > 
> > > diff --git a/introduction.tex b/introduction.tex
> > > index 979881e..3cb7a70 100644
> > > --- a/introduction.tex
> > > +++ b/introduction.tex
> > > @@ -157,5 +157,23 @@ in little-endian byte order.
> > >  in big-endian byte order.
> > >  \end{description}
> > >  
> > > +When documenting sub-byte data fields, C-like bitfield notation
> > > +is used. Fields within an integer are always listed in order,
> > > +from the least significant to the most significant bit.
> > > +
> > > +For example:
> > > +\begin{lstlisting}
> > > +be16 A : 15;
> > > +be16 B : 1;
> > > +\end{lstlisting}
> > > +documents the value A stored in the low 15 bit of a 16 bit
> > > +integer and the value B stored in the high bit of the 16 bit
> > > +integer, the integer in turn using the big-endian byte order.
> > > +
> > > +Note that this notation typically matches the way bitfields are
> > > +packed by C compilers on little-endian architectures but not the
> > > +way bitfields are packed by C compilers on big-endian
> > > +architectures.
> > > +
> > >  \newpage
> > >    
> > 
> > I must admit that this explanation confuses me a bit.  
> 
> What it is saying is that this is equivalent to
> 
> CPU_TO_BE16(B << 15 | A)
> 
> Maybe adding this part will clarify things?
> 
> 
> > Would some kind
> > of graphic representation be more helpful?  
> 
> I'm not good at graphics :)

Me neither :) But pseudo-graphics might be enough.

> 
> > For example, on s390 I would expect the structure to look like the
> > following:
> > 
> > |0  ..  14 | 15 |
> > |    A     |  B |
> > 
> > If you included another example for little-endian byte order, this
> > would clear up things more, I think.  
> 
> 
> It's BE so I think it's
> 
> | 15 |14  ..  0 |
> | B  |    A     |
> 

But that's the same, no? Or it's just IBM bitorder striking again...

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