The Performance Stop Status and Control Register (PSSCR) is used to
control the power saving facilities of the processor. This register
has various fields, some of which can be modified only in hypervisor
state, and others which can be modified in both hypervisor and
privileged non-hypervisor state. The bits which can be modified in
privileged non-hypervisor state are referred to as guest visible.

Currently the L0 hypervisor saves and restores both it's own host
value as well as the guest value of the PSSCR when context switching
between the hypervisor and guest. However a nested hypervisor running
it's own nested guests (as indicated by kvmhv_on_pseries()) doesn't
context switch the PSSCR register. That means if a nested (L2) guest
modifies the PSSCR then the L1 guest hypervisor will run with that
modified value, and if the L1 guest hypervisor modifies the PSSCR and
then goes to run the nested (L2) guest again then the L2 PSSCR value
will be lost.

Fix this by having the (L1) nested hypervisor save and restore both
its host and the guest PSSCR value when entering and exiting a
nested (L2) guest. Note that only the guest visible parts of the PSSCR
are context switched since this is all the L1 nested hypervisor can
access, this is fine however as these are the only fields the L0
hypervisor provides guest control of anyway and so all other fields
are ignored.

This could also have been implemented by adding the PSSCR register to
the hv_regs passed to the L0 hypervisor as input to the H_ENTER_NESTED
hcall, however this would have meant updating the structure layout and
thus required modifications to both the L0 and L1 kernels. Whereas the
approach used doesn't require L0 kernel modifications while achieving
the same result.

Fixes: 95a6432c ("KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests")
Cc: stable@vger.kernel.org # v4.20+
Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190703012022.15644-3-sjitindarsingh@gmail.com