KVM: s390: pv: fix external interruption loop not always detected

KVM: s390: pv: fix external interruption loop not always detected

To determine whether the guest has caused an external interruption loop
upon code 20 (external interrupt) intercepts, the ext_new_psw needs to
be inspected to see whether external interrupts are enabled.

Under non-PV, ext_new_psw can simply be taken from guest lowcore. Under
PV, KVM can only access the encrypted guest lowcore and hence the
ext_new_psw must not be taken from guest lowcore.

handle_external_interrupt() incorrectly did that and hence was not able
to reliably tell whether an external interruption loop is happening or
not. False negatives cause spurious failures of my kvm-unit-test
for extint loops[1] under PV.

Since code 20 is only caused under PV if and only if the guest's
ext_new_psw is enabled for external interrupts, false positive detection
of a external interruption loop can not happen.

Fix this issue by instead looking at the guest PSW in the state
description. Since the PSW swap for external interrupt is done by the
ultravisor before the intercept is caused, this reliably tells whether
the guest is enabled for external interrupts in the ext_new_psw.

Also update the comments to explain better what is happening.

[1] https://lore.kernel.org/kvm/20220812062151.1980937-4-nrb@linux.ibm.com/

Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Fixes: 201ae986ead7 ("KVM: s390: protvirt: Implement interrupt injection")
Link: https://lore.kernel.org/r/20230213085520.100756-2-nrb@linux.ibm.com
Message-Id: <20230213085520.100756-2-nrb@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>