KVM: PPC: Book3S HV: Don't lose pending doorbell request on migration on P9

On POWER9, when userspace reads the value of the DPDES register on a vCPU, it is possible for 0 to be returned although there is a doorbell interrupt pending for the vCPU. This can lead to a doorbell interrupt being lost across migration. If the guest kernel uses doorbell interrupts for IPIs, then it could malfunction because of the lost interrupt.

This happens because a newly-generated doorbell interrupt is signalled by setting vcpu->arch.doorbell_request to 1; the DPDES value in vcpu->arch.vcore->dpdes is not updated, because it can only be updated when holding the vcpu mutex, in order to avoid races.

To fix this, we OR in vcpu->arch.doorbell_request when reading the DPDES value.

Cc [email protected] # v4.13+

Fixes 579006944e0d ("KVM: PPC: Book3S HV: Virtualize doorbell facility on POWER9")

Signed-off-by Paul Mackerras <[email protected]>

---
  arch/powerpc/kvm/book3s_hv.c | 9 ++++++++-
  1 file changed, 8 insertions(+), 1 deletion(-)

arch/powerpc/kvm/book3s_hv.c +8 -1

index ca6c6ec..88c42e7 100644
@@ -1678,7 +1678,14 @@ static int kvmppc_get_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,
  		*val = get_reg_val(id, vcpu->arch.pspb);
  		break;
  	case KVM_REG_PPC_DPDES:
-		*val = get_reg_val(id, vcpu->arch.vcore->dpdes);
+		/*
+		 * On POWER9, where we are emulating msgsndp etc.,
+		 * we return 1 bit for each vcpu, which can come from
+		 * either vcore->dpdes or doorbell_request.
+		 * On POWER8, doorbell_request is 0.
+		 */
+		*val = get_reg_val(id, vcpu->arch.vcore->dpdes |
+				   vcpu->arch.doorbell_request);
  		break;
  	case KVM_REG_PPC_VTB:
  		*val = get_reg_val(id, vcpu->arch.vcore->vtb);