Re: [Qemu-devel] [PATCH 4/4] xen: Fix PV-on-HVM

[Stefano Stabellini]

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.


> > > >> Of course it has to do it early enough so that the guest
> > > >> doesn't crash.
> > > > 
> > > > What does early enough mean and how do we ensure that?
> > > 
> > > Early enough means that the I/O port is written very early in the boot
> > > process, even before the PCI bus is scanned by the OS.
> > > 
> > > You don't ensure it, it's up to the OS.  The OS knows whether its
> > > drivers can cope properly with surprise removal.  If they can, in
> > > principle it could write the magic value whenever it wants to.
> > 
> > Right, it is up to the OS, in general before the PCI bus is scanned.
> > In Linux we do it from hypervisor_x86->init_platform.
> 
> So early on boot you decide you want PV and so you unplug all emulated
> devices?

Yes, but only the emulated devices that can collide with PV devices:
disk and network.