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From: Pavel Fedin <p.fedin@samsung.com>
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Date: Thu, 19 Nov 2015 13:18:13 +0300
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Subject: Re: [Qemu-devel] [PATCH] virtio: Implement userspace forwarding for
	host notifiers
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To: 'Christian Borntraeger' <borntraeger@de.ibm.com>, qemu-devel@nongnu.org
Cc: 'Cornelia Huck' <cornelia.huck@de.ibm.com>, "'Michael S. Tsirkin'" <mst@redhat.com>

 Hello!

> Looks like this would be ok for virtio-ccw (as it does not call
> virtio_queue_set_host_notifier_forwarding)

 Yes, this is the reason why i intentionally did not insert extra logic into virtio_queue_set_host_notifier_fd_handler(), but made
it into separate function. I don't know whether KVM on S390 reports KVM_CAP_IOEVENTFD.

> Question is might something like this for virtio-ccw useful as well?

 Yes, provided you have something to bind userspace actions to. Looks like S390 doesn't use MMIO here at all, but some obscure (for
me) thing, called "subchannel".
 Another question is whether you can have kernel without KVM_CAP_IOEVENTFD on S390. Was it introduced since the beginning, or some
time later?

> We cannot use memory_region, though, we would need to handle that in our
> diagnose code.

 Sorry, this phrase is a complete mystery to me, i have no idea what is diagnose code, as well as have never seen S390 in real life.
I think if you have motivation, you could implement userspace forwarding for ccw yourself, since you know the thing and know how to
do it. For MMIO it was indeed very simple.

Kind regards,
Pavel Fedin
Expert Engineer
Samsung Electronics Research center Russia