Re: [Qemu-devel] [PATCH] cow: make padding in the header explicit

[Stefan Hajnoczi]

On Thu, Sep 04, 2014 at 06:07:32AM -0600, Eric Blake wrote:
> On 09/04/2014 02:58 AM, Stefan Hajnoczi wrote:
> > On-disk structures should be marked packed so the compiler does not
> > insert padding for field alignment.  Padding should be explicit so
> > on-disk layout is obvious and we don't rely on the architecture-specific
> > ABI for alignment rules.
> > 
> > The pahole(1) diff shows that the padding is now explicit and offsets
> > are unchanged:
> > 
> >     char                       backing_file[1024];   /*     8  1024 */
> >     /* --- cacheline 16 boundary (1024 bytes) was 8 bytes ago --- */
> >     int32_t                    mtime;                /*  1032     4 */
> > -
> > -   /* XXX 4 bytes hole, try to pack */
> > -
> > +   uint32_t                   padding;              /*  1036     4 */
> >     uint64_t                   size;                 /*  1040     8 */
> 
> Was a 32-bit build also inserting this padding, or do we have historical
> differences where 32-bit and 64-bit cow files are actually different,
> and we may need to be prepared to parse files from both sources?

Good point.  Let's not merge this patch since it breaks 32-bit hosts.

The fact that no one hit problems when exchanging files between 32-bit
and 64-bit machines shows that the cow format is rarely used.

At this point we have 2 different formats: one without padding
(i386-style) and one with padding (x86_64-style).  The chance of more
variants is small but who knows, maybe some other host architecture ABI
has yet another alignment rule for uint64_t.

I'd like to git rm block/cow.c but I suppose the backwards-compatible
thing to do is to introduce subformats to support both variants.
Opinions?

Stefan

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