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" <> 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 <>
> > ---
> > 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 :)
> 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 |
