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Re: [Qemu-devel] [PATCH 1/2] Vring: vring's listener's priority should h

From: liu ping fan
Subject: Re: [Qemu-devel] [PATCH 1/2] Vring: vring's listener's priority should higher than kvm
Date: Fri, 10 May 2013 14:03:34 +0800

On Thu, May 9, 2013 at 11:26 PM, Stefan Hajnoczi <address@hidden> wrote:
> 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 <address@hidden> wrote:
>> > On Thu, May 09, 2013 at 08:40:21AM +0800, Liu Ping Fan wrote:
>> >> From: Liu Ping Fan <address@hidden>
>> >>
>> >> 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?
No, it is not.

> I still don't see a kvm vs vhost race.  I see a guest vs vhost race
> which priority doesn't fix.
Yes, you are right.
The priority should be vhost > guest, and kvm > guest. So vhost == kvm
is OK. But can it be higher or why chosen as 10 not zero?

If the dependency only lies between MemoryListeners and guest, not
between listeners, then is the priority meanless?  I think we should
make sure about this, because if converting core listener to rcu
style, we will definitely break the sequence of region_add/del, ie
both add&del comes after kvm.

> Stefan

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