KVM: PPC: Book 3S: XICS: Fix potential issue with duplicate IRQ resends

It is possible that in the following order, one irq is resent twice:

        CPU 1                                   CPU 2

ics_check_resend()
  lock ics_lock
    see resend set
  unlock ics_lock
                                       /* change affinity of the irq */
                                       kvmppc_xics_set_xive()
                                         write_xive()
                                           lock ics_lock
                                             see resend set
                                           unlock ics_lock

                                         icp_deliver_irq() /* resend */

  icp_deliver_irq() /* resend again */

It doesn't have any user-visible effect at present, but needs to be avoided
when the following patch implementing the P/Q stuff is applied.

This patch clears the resend flag before releasing the ics lock, when we
know we will do a re-delivery after checking the flag, or setting the flag.
Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/kvm/book3s_hv_rm_xics.c

@@ -52,6 +52,8 @@ static void ics_rm_check_resend(struct kvmppc_xics *xics,
 		if (!state->resend)
 			continue;
 
+		state->resend = 0;
+
 		arch_spin_unlock(&ics->lock);
 		icp_rm_deliver_irq(xics, icp, state->number);
 		arch_spin_lock(&ics->lock);
@@ -400,6 +402,7 @@ static void icp_rm_deliver_irq(struct kvmppc_xics *xics, struct kvmppc_icp *icp,
 		 */
 		smp_mb();
 		if (!icp->state.need_resend) {
+			state->resend = 0;
 			arch_spin_unlock(&ics->lock);
 			goto again;
 		}

arch/powerpc/kvm/book3s_xics.c

@@ -125,6 +125,8 @@ static void ics_check_resend(struct kvmppc_xics *xics, struct kvmppc_ics *ics,
 		if (!state->resend)
 			continue;
 
+		state->resend = 0;
+
 		XICS_DBG("resend %#x prio %#x\n", state->number,
 			      state->priority);
 
@@ -155,6 +157,7 @@ static bool write_xive(struct kvmppc_xics *xics, struct kvmppc_ics *ics,
 	deliver = false;
 	if ((state->masked_pending || state->resend) && priority != MASKED) {
 		state->masked_pending = 0;
+		state->resend = 0;
 		deliver = true;
 	}
 
@@ -488,6 +491,7 @@ static void icp_deliver_irq(struct kvmppc_xics *xics, struct kvmppc_icp *icp,
 		 */
 		smp_mb();
 		if (!icp->state.need_resend) {
+			state->resend = 0;
 			arch_spin_unlock(&ics->lock);
 			local_irq_restore(flags);
 			goto again;