virtio — archive

virtio_scsi_cmd_req_cmd alignment

From: Rusty Russell <[email protected]>
To: [email protected]
Date: Tue, 29 Apr 2014 01:43:00 -0000

Stefan Hajnoczi <[email protected]> writes: > 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. Yes, let's add a note. Cheers, Rusty.