[v2 09/12] powerpc/mce: Enable MCE notifiers in external modules

[Reza Arbab]

On Thu, Jul 04, 2019 at 12:36:18PM +1000, Nicholas Piggin wrote:
>Reza Arbab's on July 4, 2019 3:20 am:
>> Since the notifier chain is actually part of the decision between (1)
>> and (2), it's a hard limitation then that callbacks be in real address
>> space. Is there any way to structure things so that's not the case?
>
>If we tested for KVM guest first, and went through and marked (maybe
>in a paca flag) everywhere else that put the MMU into a bad / non-host
>state, and had the notifiers use the machine check stack, then it
>would be possible to enable MMU here.
>
>Hmm, testing for IR|DR after testing for KVM guest might actually be
>enough without requiring changes outside the machine check handler...
>Actually no that may not quite work because the handler could take a
>SLB miss and it might have been triggered inside the SLB miss handler.
>
>All in all I'm pretty against turning on MMU in the MCE handler
>anywhere.

Hey, fair enough. Just making sure there really isnt't any room to make 
things work the way I was trying.

>> Luckily this patch isn't really necessary for memcpy_mcsafe(), but we
>> have a couple of other potential users of the notifier from external
>> modules (so their callbacks would require virtual mode).
>
>What users are there? Do they do any significant amount of logic that
>can not be moved to vmlinux?

One I had in mind was the NVIDIA driver. When taking a UE from defective 
GPU memory, it could use the notifier to save the bad address to a 
blacklist in their nvram. Not so much recovering the machine check, just 
logging before the system reboots.

The other user is a prototype driver for the IBM Research project we had 
a talk about offline a while back.

We can make this patchset work for memcpy_mcsafe(), but I think it's 
back to the drawing board for the others.

-- 
Reza Arbab