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

[Daniel P. Berrange]

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.

Daniel
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