Re: [PATCHV2 3/3] x86, ras: Add mcsafe_memcpy() function to recover from machine checks

[Andy Lutomirski]

On Fri, Dec 11, 2015 at 1:19 PM, Luck, Tony <tony.luck@xxxxxxxxx> wrote:
>> I still don't get the BIT(63) thing.  Can you explain it?
>
> It will be more obvious when I get around to writing copy_from_user().
>
> Then we will have a function that can take page faults if there are pages
> that are not present.  If the page faults can't be fixed we have a -EFAULT
> condition. We can also take machine checks if we reads from a location with an
> uncorrected error.
>
> We need to distinguish these two cases because the action we take is
> different. For the unresolved page fault we already have the ABI that the
> copy_to/from_user() functions return zero for success, and a non-zero
> return is the number of not-copied bytes.

I'm missing something, though.  The normal fixup_exception path
doesn't touch rax at all.  The memory_failure path does.  But couldn't
you distinguish them by just pointing the exception handlers at
different landing pads?

Also, would it be more straightforward if the mcexception landing pad
looked up the va -> pa mapping by itself?  Or is that somehow not
reliable?

--Andy
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