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

• From: Cornelia Huck <cohuck@redhat.com>
• To: "Michael S. Tsirkin" <mst@redhat.com>
• Date: Tue, 6 Mar 2018 09:21:30 +0100

On Mon, 5 Mar 2018 23:11:40 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Mar 05, 2018 at 05:04:40PM +0100, Cornelia Huck wrote:
> > 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?
>
> I just tried to show that B is in byte 0,
> assuming bytes are numbered 0,1,2,3 left to right.
>
> > Or it's just IBM bitorder striking again...
>
> That's what I'm saying these graphics do not help at all.
>
> It's an integer, B is the most significant bit. Integer is stored
> in BE format, thus B is the MSB in the first byte.
>
> Let me know whether writing
> CPU_TO_BE16(B << 15 | A)
>
> helps clarify things.

I think so.

Let's go with that, then. I'm probably a bad measure since I've been
subjected to the IBM notation for a long time...


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