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SubjectRe: [Xen-devel] [PATCH RFC 12/12] xen-block: implement indirect descriptors
On 28/02/13 12:19, Jan Beulich wrote:
>>>> 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?

Yes, that's not needed.

>
>> +
>> 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).

This padding is because we want to have the "id" field at the same
position as blkif_request_rw, so we need to add the padding for it to
match 32 & 64 bit blkif_request_rw structures, this prevents adding some
"if (req.op == BLKIF_OP_INDIRECT)..." if we only need to get the id of
the request.

The comment is indeed wrong, I've copied it from blkif_request_discard
and forgot to change the offset

>
> 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.

The translation is caused by the id issue described above.

> 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.





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