Comment 0 for bug 1995941

Description: KVM: PV: ext call delivered twice when receiver in PSW wait
Symptom: In a secure execution guest, the external interrupt for the
               SIGP external call order is delivered twice to a VCPU even
               though it was only sent once.
Problem: Under PV, external call interrupts are delivered by the SIGP
               interpretation facility, without KVM's involvement. But, if the
               receiving CPU is in enabled wait, KVM needs to wake the
               receiving CPU such that the interrupt can be delivered. Hence,
               in this case, the SIGP external call order causes an
               interception. In response, KVM only needs to wake the receiving
               VCPU. Interrupt delivery is then handled by the SIGP
               interpretation facility.

               KVM wrongfuly assumed it also needs to request injection for the
               external call interrupt after the respective intercept, causing
               the interrupt to be delivered twice: once through the SIGP
               interpretation facility and once through the interrupt injection
               control by KVM.
Solution: Add appropriate special handling for 108 external call
               intercepts.
Reproduction: 0. Apply kvm-unit-test submitted upstream
                  ("[kvm-unit-tests PATCH v1 0/4] s390x: add tests for SIGP
                  call orders in enabled wait").
               1. Run the smp_PV kvm-unit-test:
                    ./run_tests.sh smp_PV
               2. Check logs/smp_PV.log. If system is affected, the following
                  line can be found:
                    ABORT: smp: psw wait: ecall: Unexpected external call \
                     interrupt (code 0x1202): on cpu 1 at 0x11958
                  If the system is not affected, the line should look like
                  this:
                    PASS: smp: psw wait: ecall: received

Preventive: yes
Author: Nico Boehr <email address hidden>
Component: kernel