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Subject: Re: [virtio-dev] Re: [PATCH v8 08/16] packed virtqueues: more efficient virtqueue layout
On Tue, Feb 27, 2018 at 09:49:28AM +0800, Tiwei Bie wrote: > 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 Yes but desc_event_wrap itself is completely within the second byte. So the question of byte-order does not arise. Right, and above is also identical to: struct { __le16 desc_event_off : 15, u8 desc_event_wrap : 1; __le16 desc_event_flags : 2; }; introduction explains: \item[u8, u16, u32, u64] An unsigned integer of the specified length in bits. \item[le16, le32, le64] An unsigned integer of the specified length in bits, in little-endian byte order. > > > > > > > > > > > > +__le16 desc_event_flags : 2; /* Descriptor Event Flags */ > > > > > > +\end{lstlisting} > > > [...]
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