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Date: Tue, 13 Sep 2016 15:38:38 +0800
From: Peter Xu <peterx@redhat.com>
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Subject: Re: [Qemu-devel] [v4 4/6] hw/iommu: AMD IOMMU interrupt remapping
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To: David Kiarie <davidkiarie4@gmail.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>, Jan Kiszka <jan.kiszka@web.de>, "Michael S. Tsirkin" <mst@redhat.com>, rkrcmar@redhat.com, Eduardo Habkost <ehabkost@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Alex Williamson <alex.williamson@redhat.com>

On Mon, Sep 12, 2016 at 03:45:48PM +0300, David Kiarie wrote:
> > When we say cache here, we are mostly talking about GSI routes in
> > kernel, right? Since we still don't have other kind of interrupt
> > caches AFAIK. If so, GSI routes should already been setup even if the
> > interrupts are not triggered for a single time. So we need to
> > invalidate them even ir_cache == false.
> >
> 
> You're right but I'm not sure how to implement that while avoiding
> triggering the notifier numerous pointless times during boot.
> 
> 
> > I think the problem is why cache invalidations during boot will bug
> > the system. Any clue?
> >
> 
> The issue is not too many invalidations. I don't have a very clear idea of
> how notifiers work but I would assume it spawns a thread or they somehow
> use a multithreaded approach which would mean triggering the notifier too
> many times within a very short period many trigger a bunch of issues.

No thread is spawn, we just call the notifier callbacks.

For me it's fairly acceptable that guest sends lots of invalidations
during boot. That should not lead to any functional issues. If there
is, then something might be wrong.

I don't know whether mst will like to merge this series even without
fixing this. Anyway I would still prefer to root cause the issue, or
at least comment this out in the commit message (or codes somewhere)
so that we know there is something TBD and might cause misterious
troubles...

Thanks,

-- peterx