virtio — archive

[Stefan Hajnoczi]

On Mon, Apr 07, 2014 at 12:33:16PM +0300, Michael S. Tsirkin wrote: > I just noticed that without a pack directive, > the following is likely to cause compiler to add padding: > > struct virtio_scsi_req_cmd { > // Device-readable part > u8 lun[8]; > le64 id; > u8 task_attr; > u8 prio; > u8 crn; > u8 cdb[cdb_size]; > u8 dataout[]; > // Device-writable part > le32 sense_len; > le32 residual; > le16 status_qualifier; > u8 status; > u8 response; > u8 sense[sense_size]; > u8 datain[]; > }; > > > if cdb_size is 32 and sense_size is 96 as spec suggests, > and with no dataout and datain, padding will be added > at the tail of structure unless suppressed e.g. using > the packed directive. > > In practice this is pseudo-code, but implementations > might copy bits of this. > > That's unfortunate, I really assumed balloon is the only > instance where structure fields aren't naturally aligned. > Should we do something with it for 1.0? > Or is an implementation note in the spec enough? I never worried about this since it's not valid C but an implementation note can't hurt. Stefan