Revert series "spapr/xive: Allocate vCPU IPIs from the vCPU contexts"

Revert series "spapr/xive: Allocate vCPU IPIs from the vCPU contexts"

This series was largely built on the assumption that IPI numbers are
numerically equal to vCPU ids. Even if this happens to be the case
in practice with the default machine settings, this ceases to be true
if VSMT is set to a different value than the number of vCPUs per core.
This causes bogus IPI numbers to be created in KVM from a guest stand
point. This leads to unknow results in the guest, including crashes
or missing vCPUs (see BugLink) and even non-fatal oopses in current
KVM that lacks a check before accessing misconfigured HW (see [1]).

A tentative patch was sent (see [2]) but it seems too complex to be
merged in an RC. Since the original changes are essentially an
optimization, it seems safer to revert them for now. The damage is
done by commit acbdb9956fe9 ("spapr/xive: Allocate IPIs independently
from the other sources") but the previous patches in the series are
really preparatory patches. So this reverts the whole series:

eab0a2d06e97 ("spapr/xive: Allocate vCPU IPIs from the vCPU contexts")
acbdb9956fe9 ("spapr/xive: Allocate IPIs independently from the other sources")
fa94447a2cd6 ("spapr/xive: Use kvmppc_xive_source_reset() in post_load")
235d3b116213 ("spapr/xive: Modify kvm_cpu_is_enabled() interface")

[1] https://marc.info/?l=kvm-ppc&m=160458409722959&w=4
[2] https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg03626.html

Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Fixes: acbdb9956fe9 ("spapr/xive: Allocate IPIs independently from the other sources")
BugLink: https://bugs.launchpad.net/qemu/+bug/1900241
Signed-off-by: Greg Kurz <groug@kaod.org>
Acked-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <160554086275.1325084.12110142252189044646.stgit@bahia.lan>