[regression] Bug 221745 - PV DomU ballooning driver init issue

Hi Roger! FYI, it seems your change 0949c646d64697 ("Partial revert
"x86/xen: fix balloon target initialization for PVH dom0"") [v7.0-rc1]
caued a regression. For details see
https://bugzilla.kernel.org/show_bug.cgi?id=221745

To quote the initial comment:

> Since commit db8c4b1 (Partial revert "x86/xen: fix balloon target initialization for PVH dom0"), xen ballooning fails to init correctly on domU (on amd64 arch) with the following configuration:
> 
> -- xl.cfg
> ...
> memory      = '2048'
> maxmem      = '4096'
> ...
> --
> 
> -- domU balloon dmesg (init fails)
> [    0.908337] xen:balloon: Initialising balloon driver
> [    0.940721] Extra pages underflow current target
> --

db8c4b1 is the backport of the commit to 6.12.y; the reporter (CCed) in
a later comment to the bug tracker confirmed that the problem happens
with 7.2.0-rc3, too, and also stated:

> The dmesg by itself is not relevant except:
> [    1.105207] xen:balloon: Initialising balloon driver
> [    1.222758] ------------[ cut here ]------------
> [    1.222790] Extra pages underflow current target
> [    1.222792] WARNING: at 0xffffffff8252a424, CPU#1: swapper/0/1
> 
> init exits on ERANGE handling:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/xen/balloon.c?h=v7.2-rc4#n772
> 
> because of ERANGE:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/xen/balloon.c?h=v7.2-rc4#n714
> 
> kernel thread xen-balloon does not exist on VM with kernel newer than 6.12.75 (on 6.12 branch) where the patch was merged.

Ciao, Thorsten