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

[Prarit Bhargava]

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.

>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?

P.
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