KVM: s390: only deliver the set service event bits

The SCLP driver code masks off the last two bits of the parameter [1]
to determine if a read is required, but doesn't care about the
contents of those bits. Meanwhile, the KVM code that delivers
event interrupts masks off those two bits but sends both to the
guest, even if only one was specified by userspace [2].

This works for the driver code, but it means any nuances of those
bits gets lost. Use the event pending mask as an actual mask, and
only send the bit(s) that were specified in the pending interrupt.

[1] Linux: sclp_interrupt_handler() (drivers/s390/char/sclp.c:658)
[2] QEMU: service_interrupt() (hw/s390x/sclp.c:360..363)

Fixes: 0890ddea1a90 ("KVM: s390: protvirt: Add SCLP interrupt handling")
Signed-off-by: Eric Farman <[e-mail]>
Reviewed-by: Christian Borntraeger <[e-mail]>
Reviewed-by: Janosch Frank <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Janosch Frank <[e-mail]>
Message-Id: <[e-mail]>

arch/s390/kvm/interrupt.c

···
 		return 0;
 	}
 	ext = fi->srv_signal;
-	/* only clear the event bit */
+	/* only clear the event bits */
 	fi->srv_signal.ext_params &= ~SCCB_EVENT_PENDING;
 	clear_bit(IRQ_PEND_EXT_SERVICE_EV, &fi->pending_irqs);
 	spin_unlock(&fi->lock);
···
 	trace_kvm_s390_deliver_interrupt(vcpu->vcpu_id, KVM_S390_INT_SERVICE,
 					 ext.ext_params, 0);
 
-	return write_sclp(vcpu, SCCB_EVENT_PENDING);
+	return write_sclp(vcpu, ext.ext_params & SCCB_EVENT_PENDING);
 }
 
 static int __must_check __deliver_pfault_done(struct kvm_vcpu *vcpu)