Re: [Qemu-devel] [PATCH 0/2 v3] kvm: notify host when guest panicked

[Gleb Natapov]

On Thu, Mar 22, 2012 at 09:05:12AM +0800, Wen Congyang wrote:
> At 03/22/2012 03:19 AM, Anthony Liguori Wrote:
> > On 03/21/2012 11:25 AM, Avi Kivity wrote:
> >> On 03/21/2012 06:18 PM, Corey Minyard wrote:
> >>>
> >>>> Look at drivers/char/ipmi/ipmi_msghandler.c. It has code to send panic
> >>>> event over IMPI. The code is pretty complex. Of course if we a going to
> >>>> implement something more complex than simple hypercall for panic
> >>>> notification we better do something more interesting with it than just
> >>>> saying "panic happened", like sending stack traces on all cpus for
> >>>> instance.
> >>>
> >>> I doubt that's the best example, unfortunately.  The IPMI event log
> >>> has limited space and it has to be send a little piece at a time since
> >>> each log entry is 14 bytes.  It just prints the panic string, nothing
> >>> else.  Not that it isn't useful, it has saved my butt before.
> >>>
> >>> You have lots of interesting options with paravirtualization.  You
> >>> could, for instance, create a console driver that delivered all
> >>> console output efficiently through a hypercall.  That would be really
> >>> easy.  Or, as you mention, a custom way to deliver panic information.
> >>> Collecting information like stack traces would be harder to
> >>> accomplish, as I don't think there is currently a way to get it except
> >>> by sending it to printk.
> >>
> >> That already exists; virtio-console (or serial console emulation) can do
> >> the job.
> > 
> > I think the use case here is pretty straight forward: if the guest finds
> > itself in bad place, it wants to indicate that to the host.
> > 
> > We shouldn't rely on any device drivers or complex code.  It should be
> > as close to a single instruction as possible that can run even if
> > interrupts are disabled.
> > 
> > An out instruction fits this very well.  I think a simple protocol like:
> 
> This solution is more simple than using virtio-serial.
> 
> > 
> > inl PORT -> returns a magic number indicating the presence of qemucalls
> 
> I donot understantd this instruction's purpose.
> 
> > inl PORT+1 -> returns a bitmap of supported features
> 
> Hmm, we can execute this instruction when guest starts. If the userspace
> does not process panicked event, there is no need to notify it.
> 
> > 
> > outl PORT+1 -> data reg1
> > outl PORT+2 -> data reg2
> > outl PORT+N -> data regN
> 
> We can get the register value from vmcs. So there is no need to tell
> the register value to the host.
> 
No device should examine register value. Ideally QEMU would read
registers only during migration.

> If we decide to avoid touching hypervisor, I agree with this solution.
> 
> Thanks
> Wen Congyang
> > 
> > outl PORT -> qemucall of index value with arguments 1..N
> > 
> > Regards,
> > 
> > Anthony Liguori
> > 
> >>
> >> In fact the feature can be implemented 100% host side by searching for a
> >> panic string signature in the console logs.
> >>
> > 
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--
			Gleb.
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