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

[Anthony Liguori]

On Tue, Aug 27, 2013 at 8:13 AM, Daniel P. Berrange <address@hidden> 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.

Even if you ignore the watchdog aspect of this, having a portable
panic notifier and the ability to enhance it to include more
information (like the backtrace, etc.) is pretty darn useful.

Regards,

Anthony Liguori

> Daniel
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