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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 16:43:05 +0100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Thu, Sep 04, 2014 at 04:10:14PM +0200, Kevin Wolf wrote:
> Am 04.09.2014 um 15:51 hat Stefan Hajnoczi geschrieben:
> > 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?
>
> Can we safely detect which of the subformats we have? But I'm not sure
> if it's even worth fixing.
I think it would default to the subformat depending on the host
architecture but allow overriding with -o subformat=i386|x86_64.
I'm also not sure if it's worth fixing. The cow file format is so
rarely used I wonder if we'd be better off without it.
Stefan
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