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(a)veillard.com | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library http://libvirt.org/