Re: [Qemu-devel] [PATCH 0/2] Introduce panic hypercall

[Avi Kivity]

On 06/21/2011 11:41 AM, Gleb Natapov wrote:
> On Tue, Jun 21, 2011 at 11:09:21AM +0300, Avi Kivity wrote:
> >  On 06/21/2011 09:02 AM, Gleb Natapov wrote:
> >  >On Mon, Jun 20, 2011 at 07:34:57PM +0300, Avi Kivity wrote:
> >  >>   >The only two things which came to my mind are:
> >  >>   >
> >  >>   >    * NMI (aka. ipmitool diag) - already available in qemu/kvm - but 
> requires
> >  >>   >      in-guest kexec/kdump
> >  >>   >    * Hardware-Watchdog (also available in qemu/libvirt)
> >  >>
> >  >>   A watchdog has the advantage that is also detects lockups.
> >  >>
> >  >And has disadvantage that all time base heuristics are bite us in the
> >  >end.
> >
> >  The perf-based watchdog counts clocks-not-halted, not time, so it is
> >  safe from time issues.
> So it counts only instruction that guest actually executed? That's
> perfect then. How much overhead it has in a guest though?

Should be pretty low, especially if the guest is idle.  There were some 
negative reports about the pmu From David Ahern, so it needs to be verified.


> >                          We could make the hardware watchdog cheat in
> >  the same way.
> >
> Something like steal time, but for watchdog. But this become complicated fast.
> Watchdog emulation will have to move into kernel for starter.

Why?  You can use a performance counter from userspace.

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