On Mon, Feb 26, 2018 at 10:38:13PM +0200, Michael S. Tsirkin wrote:
> On Mon, Feb 26, 2018 at 06:51:11PM +0800, Tiwei Bie wrote:
> > On Sun, Feb 25, 2018 at 08:49:10PM +0200, Michael S. Tsirkin wrote:
> > > On Sat, Feb 24, 2018 at 01:17:08PM +0800, Tiwei Bie wrote:
> > > > On Fri, Feb 16, 2018 at 09:24:12AM +0200, Michael S. Tsirkin wrote:
> > [...]
> > > > > +\subsection{Event Suppression Structure Format}\label{sec:Basic
> > > > > +Facilities of a Virtio Device / Packed Virtqueues / Event
> > > > > Suppression Structure
> > > > > +Format}
> > > > > +
> > > > > +The following structure is used to reduce the number of
> > > > > +notifications sent between driver and device.
> > > > > +
> > > > > +\begin{lstlisting}
> > > > > +__le16 desc_event_off : 15; /* Descriptor Event Offset */
> > > > > +int desc_event_wrap : 1; /* Descriptor Event Wrap Counter */
> > > >
> > > > Is this `int` a typo?
> > >
> > > It's a single bit so I think it does not matter.
> > > What type would you like me to use instead?
> >
> > It looks a bit strange to use different types here, and
> > that's why I asked. If there is no particular reason to
> > use `int` here, maybe it's better to keep using __le16.
> >
> > Besides, just for fun. For C language, I checked gcc and
> > clang. It seems that `int desc_event_wrap:1;` is a signed
> > type. So, e.g. `p->desc_event_wrap == 1` is always false.
> >
> > Best regards,
> > Tiwei Bie
>
> I'll switch to u8 here, IMHO le16 for a single bit
> is really confusing. There's no byte order for a single byte.
Sorry, I just realized that I misunderstood your point
previously.. Just to double check, `desc_event_off` and
`desc_event_wrap` are not in the same 2 bytes?
Previously I thought both of them are in the first 2
bytes. As it said it's a structure, and the fields are
defined in a way very similar to the bit field in C.
In C,
struct {
__le16 desc_event_off : 15;
int desc_event_wrap : 1;
__le16 desc_event_flags : 2;
};
struct {
__le16 desc_event_off : 15;
__le16 desc_event_wrap : 1;
__le16 desc_event_flags : 2;
};
struct {
__le16 desc_event_off : 15,
desc_event_wrap : 1;
__le16 desc_event_flags : 2;
};
All above means `desc_event_off` and `desc_event_wrap`
are in the first 2 bytes. So I thought the `int` is a
typo. And I thought they are in the first 2 bytes (which
is little-endian).
Best regards,
Tiwei Bie
>
> > >
> > > > > +__le16 desc_event_flags : 2; /* Descriptor Event Flags */
> > > > > +\end{lstlisting}
> > [...]
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