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Subject: Re: [virtio-dev] Re: [PATCH v8 08/16] packed virtqueues: more efficient virtqueue layout
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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