[libvirt] [PATCH 3/4] xen: fix PyGrub boot device order

[Eric Blake]

On 10/12/2011 02:26 AM, Philipp Hahn wrote:
> When PyGrub is used as the bootloader in Xen, it gets passed the first
> bootable disk. Xend supports a "bootable"-flag for this, which isn't
> explicitly supported by libvirt.

Hmm, the XML has been enhanced in the meantime; we can now mark various 
disks as bootable (and in fact, in which order they should be attempted 
at boot) when targeting qemu.  Why can't the same xml changes be made to 
affect xen?  But that can be done as a separate patch.

> When converting libvirt-xml to xen-sxpr the "bootable"-flag gets
> implicitly set by xen.xend.XenConfig.device_add() for the first disk
> (marked as "Compat hack -- mark first disk bootable").
> When converting back xen-sxpr to libvirt-xml, the disks are returned in
> the internal order used by Xend ignoring the "bootable"-flag, which
> looses the original order. When the domain is then re-defined, the order

s/looses/loses/

> of disks is changed, which breaks PyGrub, since a different disk gets
> passed.
>
> When converting xen-sxpt to libvirt-xml, use the "bootable"-flag to

s/sxpt/sxpr/

> determine the first disk.
>
> This isn't perfect, since several disks can be marked as bootable using
> the Xend-API, but that is not supported by libvirt. In all known cases
> relevant to libvirt exactly one disk is marked as bootable.
>
> Signed-off-by: Philipp Hahn<hahn at univention.de>
> ---
> valgrind might complain about an uninitialized read access when copying
> disks[0] to disks[0], but that is overwritten again with the following
> assignment.

No problem from valgrind; VIR_ALLOC_N guarantees that disks starts life 
initialized at 0 (that is, calloc, not malloc).

> @@ -481,7 +485,13 @@ xenParseSxprDisks(virDomainDefPtr def,
>               if (VIR_REALLOC_N(def->disks, def->ndisks+1)<  0)
>                   goto no_memory;
>
> -            def->disks[def->ndisks++] = disk;
> +            /* re-order disks if there is a bootable device */
> +            if (STREQ_NULLABLE(bootable, "1")) {
> +                def->disks[def->ndisks++] = def->disks[0];
> +                def->disks[0] = disk;
> +            } else {
> +                def->disks[def->ndisks++] = disk;
> +            }

Feels a bit hacky, compared to a memmove to slide all disks; but if, as 
you say, xend already returns disks in an arbitrary order, then it 
doesn't matter if we preserve order.

ACK and pushed.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org