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SubjectRe: [Xen-devel] [PATCH RFC 12/12] xen-block: implement indirect descriptors
>>> On 28.02.13 at 13:00, Roger Pau Monné<roger.pau@citrix.com> wrote:
> 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.

Oh, right, that's desirable of course.

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

But the offset stated there then is right after all - I forgot that
there is a 1-byte field outside the union (the way this is being done
in the upstream Linux header is really ugly imo, but I guess Jeremy
and/or Konrad liked it that way). That's also why the packed
attribute is needed here.

But you will probably want to switch sector_number and handle, so
that sector_number becomes aligned, and add another 16-bit
padding field between handle and indirect_grefs[].

I also wonder whether "indirect_op" wouldn't better be named
"actual_op" or just "op".

Jan
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