Re: [virtio] virtio_scsi_cmd_req_cmd alignment

[Stefan Hajnoczi <stefanha@redhat.com>]

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