Re: [Qemu-devel] [libvirt] pvpanic plans?

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Aug 27, 2013 at 02:13:34PM +0100, Daniel P. Berrange wrote:
> On Thu, Aug 22, 2013 at 09:16:57PM +0200, Paolo Bonzini wrote:
> > Il 22/08/2013 19:53, Laszlo Ersek ha scritto:
> > >> > We should just introduce a simple watchdog device based on virtio and
> > >> > call it a day.  Then it's cross platform, solves the guest enumeration
> > >> > problem, and libvirt can detect the presence of the new device.
> > > If the guest doesn't initialize the proposed virtio-panic device, then
> > > it will lie dormant too, just like the current pvpanic device. That's good.
> > > 
> > > However a new (standalone) virtio device will take up yet another PCI
> > > function (a full device if you want it to be hotpluggable). PCI
> > > functions are scarcer than ioports.
> > 
> > Not just that.  Panic notifiers are called in a substantially unknown
> > environment, with locks taken or interrupts already set up.
> > 
> > This is why we went for a simple ISA device.  Configuration via ACPI
> > follows naturally from there, and anyway any other standard of the day
> > would have had the same problem with Windows.  At some point we had ACPI
> > methods instead of a simple ioport write, but we had to remove that
> > because the ACPI subsystem might have had its lock taken.
> > 
> > Also, a virtio watchdog device makes little sense, IMHO.  PV makes sense
> > if emulation has insufficient performance, excessive CPU usage, or
> > excessive complexity.  We already have both an ISA and a PCI watchdog,
> > and they serve their purpose wonderfully.
> 
> I also don't think that panic notifiers & watchdogs are really
> serving the same purpose. The panic notifier is an alert to a
> specific known kernel crash. A watchdog is merely a timeout,
> which is inferred to mean /something/ went wrong. Both have
> their uses IMHO & we should not conflate the two.

Exactly this.  They are two different things.

Of course ILOs combine both (and more) in one mega-device :-)

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/