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Subject: Re: [virtio-comment] [PATCH 1/1] ccw: ccw payload description
Cornelia Huck <cornelia.huck@de.ibm.com> writes: > On Mon, 20 Jan 2014 11:17:17 +0200 > "Michael S. Tsirkin" <mst@redhat.com> wrote: > >> On Mon, Jan 20, 2014 at 02:28:00PM +1030, Rusty Russell wrote: >> > We'll just have to live with the lack of padding, and let's try not to >> > add any more packed-requiring structures. If we deprecate >> > virtio-balloon, the rest will be contained in CCW. >> > >> > Cheers, >> > Rusty. >> >> For CCW, how about my idea of adding optional padding explicitly? >> In the compatibility section, we can explicitly specify the >> the length. >> Drivers can use offsetof(padding) and avoid packed attribute. > > Since Rusty does not like the tables, this looks like a good idea. Surely making the padding optional makes it more complex, for no real gain? It's not that much simpler in practice, is it? This patch just removes the __attribute__((packed)) where it's useless, and notes explicitly where it's required: diff --git a/content.tex b/content.tex index 803615d..539bf68 100644 --- a/content.tex +++ b/content.tex @@ -1992,7 +1992,7 @@ about a queue. It uses the following structure for communicating: struct vq_config_block { be16 index; be16 max_num; - } __attribute__ ((packed)); + }; \end{lstlisting} The requested number of buffers for queue index is returned in @@ -2010,7 +2010,7 @@ structure is be16 num; be64 avail; be64 used; - } __attribute__ ((packed)); + }; \end{lstlisting} desc, avail and used contain the guest addresses for the descriptor table, @@ -2029,7 +2029,7 @@ CCW_CMD_SET_VQ uses the following communication block: be32 align; be16 index; be16 num; - } __attribute__ ((packed)); + }; \end{lstlisting} queue contains the guest address for queue index, num the number of buffers @@ -2071,7 +2071,8 @@ for a total of 8192 feature bits. Feature bits are in little-endian byte order. The CCW commands dealing with features use the following -communication block: +communication block. Note that this command is exactly +5 bytes long, violating the normal C structure padding rules: \begin{lstlisting} struct virtio_feature_desc { @@ -2159,7 +2160,8 @@ two stages: To communicate the location of the summary and queue indicator bits, the driver uses the CCW_CMD_SET_IND_ADAPTER command with the following -payload: +payload. Note that this command is exactly 25 bytes long, violating +the normal C structure padding rules: \begin{lstlisting} struct virtio_thinint_area {
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