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Date: Thu, 9 May 2013 17:26:42 +0200
From: Stefan Hajnoczi <stefanha@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] Vring: vring's listener's priority
 should higher than kvm
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To: liu ping fan <qemulist@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, "Michael S. Tsirkin" <mst@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Anthony Liguori <anthony@codemonkey.ws>, Jan Kiszka <jan.kiszka@siemens.com>, Paolo Bonzini <pbonzini@redhat.com>

On Thu, May 09, 2013 at 05:00:20PM +0800, liu ping fan wrote:
> On Thu, May 9, 2013 at 4:30 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > On Thu, May 09, 2013 at 08:40:21AM +0800, Liu Ping Fan wrote:
> >> From: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> >>
> >> Hosts threads which handle vring should have high MemoryListener priority
> >> than kvm. For currently code, take the following scenario:
> >>   kvm_region_add() run earlier before vhost_region_add(), then in guest,
> >> vring's desc[i] can refer to addressX in the new region known by guest.
> >> But vhost does not know this new region yet, and the vring handler will
> >> fail.
> >
> > Is there a concrete scenario where this happens?
> >
> > I can think of situations like the ioeventfd being readable before
> > vhost/hostmem is populated.  But I don't see how that's related to the
> > priority of kvm_region_add().
> >
> For kvm, ie, In guest, vring_desc.addr can point to a chunk of data in
> the new added memory, and kick vhost. The vhost has not added this new
> region, so its local lookup table can not translate this new address,
> and vring handler will fail.  If vhost priority is higher than kvm,
> then, it will know this new address earlier than kvm.

Isn't the real solution to ensure that the memory API is up-to-date
before we notify the guest of memory hotplug?

I still don't see a kvm vs vhost race.  I see a guest vs vhost race
which priority doesn't fix.

Stefan