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

[Stefan Hajnoczi <stefanha@redhat.com>]

On Fri, May 10, 2013 at 02:03:34PM +0800, liu ping fan wrote:
> On Thu, May 9, 2013 at 11:26 PM, Stefan Hajnoczi <stefanha@redhat.com> 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 <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?
> >
> 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.

Okay, so now we're left with the question "what are the ordering
dependencies between memory listeners?".

I poked around with git-blame(1) but didn't find an explanation.  The
best I can come up with is that the core listeners in exec.c update
QEMU's guest RAM and I/O port mappings, kvm/vhost/xen should be able to
query them.  Therefore exec.c listeners have priority 0 or 1.

BTW the commit that introduced priorities is:

  commit 72e22d2fe17b85e56b4f0c437c61c6e2de97b308
  Author: Avi Kivity <avi@redhat.com>
  Date:   Wed Feb 8 15:05:50 2012 +0200

      memory: switch memory listeners to a QTAILQ

Stefan