Re: [PATCH v3 0/5] kvm: Make ioeventfd usable on s390.

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Feb 26, 2013 at 01:04:21PM +0200, Michael S. Tsirkin wrote:
> On Mon, Feb 25, 2013 at 04:27:45PM +0100, Cornelia Huck wrote:
> > Here's the latest version of my patch series enabling ioeventfds
> > on s390, again against kvm-next.
> > 
> > Patches 1 and 2 (cleaning up initialization and exporting the virtio-ccw
> > api) would make sense even independent of the ioeventfd enhancements.
> > 
> > Patches 3-5 are concerned with adding a new type of ioeventfds for
> > virtio-ccw notifications on s390. The naming is now hopefully clearer.
> > We won't add ioeventfd support for the legacy s390-virtio transport.
> > 
> > Please consider applying.
> 
> I just had a thought: this makes us lookup the device on the bus
> for each notification. It would be better to simply get the
> device index from guest instead.
> 
> We could validate that it matches the correct device,
> if not - fallback to the current linear scan.
> 
> We could return the index to guest for the next call.
> 
> I know this needs guest changes but it's still not too late to
> fix this for 3.9 guests so that we won't need to worry
> about compatibility going forward.
> 
> Hmm?

And just to clarify, here's what I mean (BTW, why doesn't
this code use the interfaces from kvm_para.h?)
I think it's a good idea to merge this before 3.9 so we don't
need to worry about legacy going forward.

Completely untested, just to give you the idea.

--->
virtio_ccw: pass a cookie value to kvm hypercall

Lookups by channel/vq pair on host during virtio notifications might be
expensive.  Interpret hypercall return value as a cookie which host can
use to do device lookups for the next notification more efficiently.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---

diff --git a/drivers/s390/kvm/virtio_ccw.c b/drivers/s390/kvm/virtio_ccw.c
index 2029b6c..1054f3a 100644
--- a/drivers/s390/kvm/virtio_ccw.c
+++ b/drivers/s390/kvm/virtio_ccw.c
@@ -77,6 +77,7 @@ struct virtio_ccw_vq_info {
 	void *queue;
 	struct vq_info_block *info_block;
 	struct list_head node;
+	long cookie;
 };
 
 #define KVM_VIRTIO_CCW_RING_ALIGN 4096
@@ -145,15 +146,18 @@ static int ccw_io_helper(struct virtio_ccw_device *vcdev,
 }
 
 static inline long do_kvm_notify(struct subchannel_id schid,
-				 unsigned long queue_index)
+				 unsigned long queue_index,
+				 long cookie)
 {
 	register unsigned long __nr asm("1") = KVM_S390_VIRTIO_CCW_NOTIFY;
 	register struct subchannel_id __schid asm("2") = schid;
 	register unsigned long __index asm("3") = queue_index;
 	register long __rc asm("2");
+	register long __cookie asm("4") = cookie;
 
 	asm volatile ("diag 2,4,0x500\n"
-		      : "=d" (__rc) : "d" (__nr), "d" (__schid), "d" (__index)
+		      : "=d" (__rc) : "d" (__nr), "d" (__schid), "d" (__index),
+		      "d"(__cookie)
 		      : "memory", "cc");
 	return __rc;
 }
@@ -166,7 +170,7 @@ static void virtio_ccw_kvm_notify(struct virtqueue *vq)
 
 	vcdev = to_vc_device(info->vq->vdev);
 	ccw_device_get_schid(vcdev->cdev, &schid);
-	do_kvm_notify(schid, virtqueue_get_queue_index(vq));
+	info->cookie = do_kvm_notify(schid, virtqueue_get_queue_index(vq), info->cookie);
 }
 
 static int virtio_ccw_read_vq_conf(struct virtio_ccw_device *vcdev,