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

Reza Arbab's on July 5, 2019 12:50 pm:

On Thu, Jul 04, 2019 at 12:36:18PM +1000, Nicholas Piggin wrote:

quoted

Reza Arbab's on July 4, 2019 3:20 am:

quoted

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.

Understand.

quoted

quoted

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.

Okay. It might be possible to save the address in the kernel and
then notify the driver afterward. For user-mode and any non-atomic
user copy AFAIK the irq_work should practically run synchronously
after the machine check returns so it might be enough to have a
notifier in the irq work processing.

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

For the first stage that would be preferable.

Thanks,
Nick