[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH] cow: make padding in the header explicit
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH] cow: make padding in the header explicit |
Date: |
Thu, 4 Sep 2014 14:51:32 +0100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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
pgp0K2rWz43Bh.pgp
Description: PGP signature