Re: Reset problem vs. MMIO emulation, hypercalls, etc...

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2012-08-03 at 15:05 -0300, Marcelo Tosatti wrote:

> See kvm_arch_process_async_events() call to qemu_system_reset_request()
> in target-i386/kvm.c.
> 
> The whole thing is fragile, though: we rely on the order events
> are processed inside KVM_RUN, in x86:
> 
> 1) If there is pending MMIO, process it.
> 2) If not, return with -EINTR (and KVM_EXIT_INTR) in case
> there is a signal pending.
> 
> That way, the vcpu will not process the stop event from the main loop
> (ie not exit from the kvm_cpu_exec() loop), until MMIO is finished.

Right, it is fragile, thankfully we appear to adhere to the same
ordering on powerpc so far :-)

So we'll need to test but it looks like we might be able to fix our
problem without a kernel or API change, just by changing qemu to
do the same exit_request trick for our reboot hypercall.

Long run however, I wonder whether we should consider an explicit ioctl
to complete those pending operations instead...

Cheers,
Ben.