Re: [Qemu-devel] [PATCH 1/1] parallels: add format spec

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

On 18.11.2015 09:39, Eric Blake wrote:
> On 11/17/2015 11:15 PM, Vladimir Sementsov-Ogievskiy wrote:
>
>>> You may want to compare to the qcow2 spec, which also lists expected
>>> byte offsets for each field (rather than having to count how many
>>> earlier fields of which widths were specified).
>> I've compared qed spec with qcow2 spec and like the first one. What the
>> need of specifying each offset? Creating a c-structure is simpler when
>> you see types. and than offsetof and sizeof may be used if needed.
>> Nobody will #define numeric offsets I think.
> You'd be surprised (libvirt has some hard-coded numeric offsets:
> https://libvirt.org/git/?p=libvirt.git;a=blob;f=src/util/virstoragefile.c;h=2aa1d90;hb=5ed7afa9d#l159)
>
> Also, listing offsets makes it obvious that you are NOT relying on
> compiler padding, and makes it obvious whether you have been careful
> that all 64-bit quantities are 8-byte-aligned without wasting space.
>
> The original cow format (not qcow or qcow2) has the awful distinction of
> NOT having specified offsets, and had different layouts on 32-bit
> platforms than it did on 64-bit platforms; hence, we retired it in
> commit 550830f9 as unsupportable.
>

Thanks for explanation, I'll rework this. May be, it's definitely better 
to be absolutely accurate than get more comfort, when it is about 
specification..

-- 
Best regards,
Vladimir
* now, @virtuozzo.com instead of @parallels.com. Sorry for this inconvenience.