Re: [PATCH v5 2/2] KVM: PPC: Exit guest upon MCE when FWNMI capability is enabled

[Paul Mackerras <paulus@ozlabs.org>]

On Wed, Jan 18, 2017 at 11:19:26AM +0530, Mahesh Jagannath Salgaonkar wrote:
> On 01/16/2017 10:05 AM, Paul Mackerras wrote:
> > On Fri, Jan 13, 2017 at 04:51:45PM +0530, Aravinda Prasad wrote:

[snip]

> >>  	case BOOK3S_INTERRUPT_MACHINE_CHECK:
> >> +		/* Exit to guest with KVM_EXIT_NMI as exit reason */
> >> +		run->exit_reason = KVM_EXIT_NMI;
> >> +		r = RESUME_HOST;
> >>  		/*
> >> -		 * Deliver a machine check interrupt to the guest.
> >> -		 * We have to do this, even if the host has handled the
> >> -		 * machine check, because machine checks use SRR0/1 and
> >> -		 * the interrupt might have trashed guest state in them.
> >> +		 * Invoke host-kernel handler to perform any host-side
> >> +		 * handling before exiting the guest.
> >>  		 */
> >> -		kvmppc_book3s_queue_irqprio(vcpu,
> >> -					    BOOK3S_INTERRUPT_MACHINE_CHECK);
> >> -		r = RESUME_GUEST;
> >> +		kvmppc_machine_check_hook();
> > 
> > Note that this won't necessarily be called on the same CPU that
> > received the machine check.  This will be called on thread 0 of the
> > core (or subcore), whereas the machine check could have occurred on
> > some other thread.  Are you sure that the machine check handling code
> > will be OK with that?
> 
> That will have only one problem. get_mce_event() from
> opal_machine_check() may not be able to pull mce event for error on
> non-zero thread. We should hook the mce event into vcpu structure during
> kvmppc_realmode_machine_check() and then pass it to
> ppc_md.machine_check_exception() as an additional argument.

To move things along...

Mahesh, how would we get hold of the mce event from real-mode assembly
code?  What function would we need to call to get the event?  Could
you write some code (or at least some pseudo-code) to illustrate how
it would be done?

Thanks,
Paul.