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From: Pavel Fedin <p.fedin@samsung.com>
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Date: Thu, 19 Nov 2015 15:17:57 +0300
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Subject: Re: [Qemu-devel] [PATCH] virtio: Implement userspace forwarding for
 host notifiers
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To: 'Paolo Bonzini' <pbonzini@redhat.com>, "'Michael S. Tsirkin'" <mst@redhat.com>
Cc: 'Cornelia Huck' <cornelia.huck@de.ibm.com>, 'Christian Borntraeger' <borntraeger@de.ibm.com>, qemu-devel@nongnu.org

 Hello!

> > Paolo, do you see anything wrong with making
> > memory_region_add_eventfd work (slowly) without kvm ioeventfd support?
> >
> 
> Sure, it's even been on the todo list for a while.  Stefan Hajnoczi had
> a patch, the only ugly thing was that it slowed down a little all
> non-ioeventfd accesses, but I guess that's okay because it's probably
> not measurable.
> 
> http://lists.nongnu.org/archive/html/qemu-devel/2015-07/msg04710.html

 Ok, good. So, this has already been tested. Ok, i think i can further extend this patch by:
1. Add a flag to disable memory_region_dispatch_write_ioeventfds(), to be set by KVM code for example. So that with KVM we have even
smaller performance degradation.
2. Issue a one-shot warning, like in my current patch, so that the user knows that his/her kernel prevents from getting the best
performance.
 Will it be OK then?

 Cc'ed also Cornelia because he's taking part in CCW discussion. And the same kind of generic event forwarding could be implemented
for CCW then.

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