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

[Stefano Stabellini]

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.


> >> 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.