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Date: Fri, 12 Apr 2013 10:41:06 +0200
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20130412084106.GD31055@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 3/5] vring: use hostmem's RAM safe api
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To: liu ping fan <qemulist@gmail.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Anthony Liguori <aliguori@us.ibm.com>, Jan Kiszka <jan.kiszka@siemens.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org, Vasilis Liaskovitis <vasilis.liaskovitis@profitbricks.com>, Paolo Bonzini <pbonzini@redhat.com>

On Fri, Apr 12, 2013 at 12:49:45PM +0800, liu ping fan wrote:
> On Thu, Apr 11, 2013 at 6:15 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > On Mon, Apr 01, 2013 at 04:20:32PM +0800, Liu Ping Fan wrote:
> >> @@ -51,7 +50,7 @@ bool vring_setup(Vring *vring, VirtIODevice *vdev, int n)
> >>
> >>  void vring_teardown(Vring *vring)
> >>  {
> >> -    hostmem_finalize(&vring->hostmem);
> >> +    memory_region_unref(vring->vring_mr);
> >>  }
> >
> > dataplane keeps a reference to the vring.  This prevents memory hot
> > unplug while the device is up.  If this is a problem we'll have to
> > reduce the lifespan of the vring mapping.
> >
> hot unplug is rare case, maybe we can ignore the delay of device's finalize.

I thought about the vring more and I think we can keep the current
behavior.  dataplane keeps the vring up when the device is active.

When the guest kernel/device driver resets the device then we unmap the
vring.  It's reasonable that the guest needs to release the virtio
device before hot unplugging memory used for the vring :).

Therefore, in practice this behavior should be fine.

Stefan