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Re: [Qemu-devel] [PATCH v2 2/3] virtio-mmio: introduce set_guest_notifie

From: Pavel Fedin
Subject: Re: [Qemu-devel] [PATCH v2 2/3] virtio-mmio: introduce set_guest_notifiers
Date: Fri, 08 May 2015 13:30:18 +0300


> Yes, I think it makes sense to just pick the low-hanging fruit for
> virtio-mmio and wait for pci.

 Does this mean that my series can be accepted as it is? Since PCI is 
potentially better
solution, MMIO is a low priority in my project, and i have lots of other tasks. 
This means
i unfortunately don't have time for further refactor. If you ACK, i will resend 
the series
once again as v3, i set up git send-email and it should be working now.
 I just wanted to share this piece because it's already done, and i would not 
like it to
go to oblivion again.

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

> -----Original Message-----
> From: address@hidden [mailto:qemu-devel-
> address@hidden On Behalf Of Cornelia Huck
> Sent: Friday, May 08, 2015 1:20 PM
> To: Pavel Fedin
> Cc: address@hidden
> Subject: Re: [Qemu-devel] [PATCH v2 2/3] virtio-mmio: introduce 
> set_guest_notifiers
> On Fri, 08 May 2015 09:45:00 +0300
> Pavel Fedin <address@hidden> wrote:
> >  Hello!
> >
> > > Hm, weren't there some patches for irqfd on arm?
> >
> >  Yes, there were. However, they had a design problem by breaking backwards
> > with unmodified virtio. Their idea was to set up one more shared memory 
> > area between
> > virtio and vhost-net and use it to pass ISR value, which helps to 
> > distinguish, which
> > took place (queue update or config change). So, this idea was rejected.
> >  http://lists.gnu.org/archive/html/qemu-devel/2014-10/msg03056.html
> >
> >  I thought about it, and technically, i think, this can be done in better 
> > way.
> > as far as i understood, all we need is mechanism for distinguishing between 
> > these two
> > events. On PCI we do this by using multiple IRQs via MSI-X, and one IRQ 
> > signals
> > one type of event. MSI-X code also has "two IRQs" mode as a failsafe, where 
> > one IRQ
> > signals config change and another IRQ signals queues update (and all queues 
> > are polled
> > turn). I think a similar thing could be done for virtio-mmio. It could 
> > allocate two
> > instead of one and describe both of them in the device tree. Guest side, 
> > upon seeing
> > could make use of those two IRQs and acknowledge to the host side that 
> > "yes, i am new
> > version and use new mode".
> >  But, sorry, i will unlikely implement this, because we already have PCI 
> > with MSI-X (i
> > hope this is going to be published soon), so my project can use PCI 
> > emulation. So
> > implementing irqfds for virtio-mmio is a bit out of my scope.
> Thanks for the explanation.

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