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

Michael S. Tsirkin
2013-02-26T13:41:56Z

On Tue, Feb 26, 2013 at 01:13:39PM +0100, Christian Borntraeger wrote:
> On 26/02/13 12:18, Michael S. Tsirkin wrote:
> > 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,
>
>
> Hmmm, forget my last mail. This actually could be even forward and backward compatible.
> In the virtio spec we will not define the cookie format (just 64bit int). That will allow
> qemu or future kernels to use that for other things (as long as a validity check is
> possible) if we dont have a kvm bus.
>
> Now:
>
> old guest, old host:
> works.
>
> old guest, new host:
> the cookie from the guest contains junk, the host needs to detect that the cookie is
> junk and ignores it. It will return the new cookie anyway.
>
> new guest, old host:
> The guest will get a junk cookie and pass it back to the host. But the host will ignore
> it anyway.
>
> new guest, new host:
> works.
>
> So...
> Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>

So let's apply the patch for 3.9 and avoid caring about
"old guests" much?

--
MST