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Date: Wed, 16 May 2012 16:24:15 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 4/4] xen: Fix PV-on-HVM
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To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Anthony Perard <anthony.perard@citrix.com>, Paolo Bonzini <pbonzini@redhat.com>, QEMU-devel <qemu-devel@nongnu.org>, Anthony Liguori <anthony@codemonkey.ws>, Xen Devel <xen-devel@lists.xen.org>

On Wed, May 16, 2012 at 11:37:02AM +0100, Stefano Stabellini wrote:
> On Wed, 16 May 2012, Michael S. Tsirkin wrote:
> > On Wed, May 16, 2012 at 11:19:53AM +0100, Stefano Stabellini wrote:
> > > On Wed, 16 May 2012, Paolo Bonzini wrote:
> > > > Il 16/05/2012 10:13, Michael S. Tsirkin ha scritto:
> > > > > On Wed, May 16, 2012 at 10:06:25AM +0200, Paolo Bonzini wrote:
> > > > >> On Xen the PV drivers can ask the firmware to surprise-remove the
> > > > >> emulated NICs.
> > > > > 
> > > > > So driver tells firmware (meaning acpi? how?) that it's ok
> > > > > to do surprize removal?
> > > > 
> > > > It writes something to some I/O port, and then QEMU surprise-removes the
> > > > NICs.
> > > 
> > > Yes, writing to a static I/O port provided by the Xen platform PCI
> > > device, see hw/xen_platform.c:platform_fixed_ioport_writew.
> > > 
> > > The guest can ask to unplug emulated NICs and disks this way.
> > > Surprise-removal is OK in these cases.
> > 
> > Confused.
> > Don't you want to just remove the device on unplug?
> 
> Yes, the NIC needs to "disappear" from the PCI bus.
> 
> 
> > In fact the equivalent of guest calling _EJ0?
> 
> Except that _EJ0 can or cannot be implemented, while this doesn't have
> to go through ACPI or PCI hotplug and it is supposed to always work.

So the answer is to simply free on unplug exactly
like _EJ0 does. There's no forcing and no surprise removal here.

-- 
MST