Re: [Xen-devel] [PATCH RFC 12/12] xen-block: implement indirect descriptors

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Thu, Feb 28, 2013 at 01:28:48PM +0000, Jan Beulich wrote:
> >>> 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.

I am not particularly found as I keep on forgetting about the 1-byte field
as well. If you have a patch to clean it up would love to see it.

> 
> 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".

<nods> 'op' sounds good. With a comment saying it can do all of the BLKIF_OPS_..
except the BLKIF_OP_INDIRECT one. Thought one could in theory chain
it that way for fun.

> 
> Jan