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Re: [libvirt] [PATCH] migrate VMs between different-endian hosts



On Mon, Apr 11, 2011 at 11:46:55AM -0600, Eric Blake wrote:
> On 04/11/2011 07:36 AM, Daniel Veillard wrote:
> >>>> @@ -3097,6 +3107,11 @@ qemuDomainSaveImageOpen(struct qemud_dri
> >>>>      }
> >>>>
> >>>>      if (header.version>  QEMUD_SAVE_VERSION) {
> >>>> +        /* convert endianess and try again */
> >>>> +        bswap_header(&header);
> >>>> +    }
> >>>   Hum, isn't there a more reliable way to detect the change of
> >>> endianness ? That's a bit fishy IMHO :-)
> >> The problem is that the header should not have been written in a
> >> hosts' native format. So what can go wrong? QEMUD_SAVE_VERSION is
> >> '2'. Either we find 1 or 2 here and go ahead and accept it 'as-is'.
> >> Otherwise anything bigger than 3 is not accepted and swapped. 3 then
> >> becomes 0x03 00 00 00 and is discarded. 0x 02 00 00 00 would be
> >> swapped to '2' and accepted.
> > 
> >   yeah, I understand, okay,
> 
> Should we be writing the header in a particular byte order, regardless
> of host endianness?  Or does that require bumping the header version to
> 3 anyways?

  The thing I'm worried about is that by bumping older versions won't
be able to restore the new dumps, and that could be a serious issue on
shared storage. We are not introducing a new feature, so I thin we need
to preserve forward compatibility. The patch as is is minimal but
achieves it.

Daniel

-- 
Daniel Veillard      | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
daniel veillard com  | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library  http://libvirt.org/


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