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

[Gleb Natapov]

On Wed, Mar 21, 2012 at 02:19:34PM -0500, 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:
> 
> inl PORT -> returns a magic number indicating the presence of qemucalls
> inl PORT+1 -> returns a bitmap of supported features
> 
Sigh, one more PV isa device.

> outl PORT+1 -> data reg1
> outl PORT+2 -> data reg2
> outl PORT+N -> data regN
> 
> outl PORT -> qemucall of index value with arguments 1..N
And you think you can trust panicked SMP guest to not call this on
multiple cpus simultaneously?

> 
> Regards,
> 
> Anthony Liguori
> 
> >
> >In fact the feature can be implemented 100% host side by searching for a
> >panic string signature in the console logs.
> >

--
                        Gleb.