Re: [PATCH v2 1/2] x86: mce: kexec: turn off MCE in kexec

[Naoya Horiguchi]

On Fri, Feb 27, 2015 at 08:14:47AM -0500, Prarit Bhargava wrote:
> On 02/27/2015 07:46 AM, Naoya Horiguchi wrote:
> > Hi Prarit,
> > 
> > On Fri, Feb 27, 2015 at 06:09:52AM -0500, Prarit Bhargava wrote:
> > ...
> >> > @@ -157,6 +160,11 @@ void native_machine_crash_shutdown(struct pt_regs *regs)
> >> >      /* The kernel is broken so disable interrupts */
> >> >      local_irq_disable();
> >> >
> >> > +    /*
> >> > +     * We can't expect MCE handling to work any more, so turn it off.
> >> > +     */
> >> > +    cpu_emergency_mce_disable();
> >>
> >> What if the system is actually having problems with MCE errors -- which are
> >> leading to system panics of some sort.  Do you *really* want the system to
> >> continue on at that point?
> > 
> > Yes, when running the above code, the system doesn't run any business logic,
> > so no worry about consuming broken data caused by HW errors.
> > And what we really want to get is any kind of information to find out what
> > caused the 1st panic, which are likely to be contained in kdump data.
> > So I think it's justified to improve the success rate of kdump by continuing
> > the operation here.
> 
> I looked into it a bit further -- IIUC (according to the Intel spec) disabling
> MCE this way will result in power cycle of the system if an MCE is detected.  So
> I guess it isn't a worry for Intel.  If anyone from AMD can hazard a guess what
> happens in their case it would be appreciated.
> 
> I still don't like this approach all that much as a corrected non-fatal error is
> something I would want to know about as an admin, but that risk is mitigated by
> BMC and system monitoring hardware.

Generally corrected non-fatal errors are not reported via MCE but via CMCI,
so not affected by sync timeout problem (they should be logged by mcelog after
reboot in a normal manner.)

But as for BMC/FW/HW logging, I'm not 100% sure that such logging mechanisms
still work when disabling CR4.MCE, so I might need reconsider this approach.

> >But the MCE handler is still enabled after that, so
> >if MCE happens and broadcasts around CPUs after the main thread starts the
> >2nd kernel (which might not start MCE yet, or might decide not to start MCE,)
> >MCE handler runs only on the other CPUs (not on the main thread,) leading to
> >kernel panic with MCE synchronization.
> 
> Not having looked at the code (and relying on your description) -- there is no
> way to disable the MCE handler?

We have the way, as Tony suggesting in another email. And I feel like moving
back to that approach in the next version.

Thanks,
Naoya Horiguchi--
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