lkml.org 
[lkml]   [2013]   [Feb]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [Xen-devel] [PATCH RFC 12/12] xen-block: implement indirect descriptors
>>> On 28.02.13 at 11:28, Roger Pau Monne <roger.pau@citrix.com> wrote:
> @@ -109,6 +111,16 @@ typedef uint64_t blkif_sector_t;
> */
> #define BLKIF_MAX_SEGMENTS_PER_REQUEST 11
>
> +#define BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST 8
> +
> +struct blkif_request_segment_aligned {
> + grant_ref_t gref; /* reference to I/O buffer frame */
> + /* @first_sect: first sector in frame to transfer (inclusive). */
> + /* @last_sect: last sector in frame to transfer (inclusive). */
> + uint8_t first_sect, last_sect;
> + uint16_t _pad; /* padding to make it 8 bytes, so it's cache-aligned */
> +} __attribute__((__packed__));

What's the __packed__ for here?

> +
> struct blkif_request_rw {
> uint8_t nr_segments; /* number of segments */
> blkif_vdev_t handle; /* only for read/write requests */
> @@ -138,11 +150,24 @@ struct blkif_request_discard {
> uint8_t _pad3;
> } __attribute__((__packed__));
>
> +struct blkif_request_indirect {
> + uint8_t indirect_op;
> + uint16_t nr_segments;
> +#ifdef CONFIG_X86_64
> + uint32_t _pad1; /* offsetof(blkif_...,u.indirect.id) == 8 */
> +#endif

Either you want the structure be packed tightly (and you don't care
about misaligned fields), in which case you shouldn't need a padding
field. That's even more so as there's no padding between indirect_op
and nr_segments, so everything is misaligned anyway, and the
comment above is wrong too (offsetof() really ought to yield 7 in
that case).

Or you want the structure fields aligned, in which case you again
ought to drop the use of the __packed__ attribute and introduce
_all_ necessary padding fields.

> + uint64_t id;
> + blkif_vdev_t handle;
> + blkif_sector_t sector_number;
> + grant_ref_t indirect_grefs[BLKIF_MAX_INDIRECT_GREFS_PER_REQUEST];
> +} __attribute__((__packed__));

And then it would be quite nice for new features to no longer
require translation between a 32- and a 64-bit layout at all.

Plus, rather than introducing uninitialized padding fields, I'd
suggest using fields that are required to be zero initialized, to
allow giving them a meaning later.

Jan



\
 
 \ /
  Last update: 2013-02-28 12:41    [W:0.214 / U:0.136 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site