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

From: Andy Lutomirski
Date: Fri Dec 11 2015 - 17:56:11 EST

On Fri, Dec 11, 2015 at 2:45 PM, Luck, Tony <tony.luck@xxxxxxxxx> wrote:
>>> But a machine check safe copy_from_user() would be useful
>>> current generation cpus that broadcast all the time.
>> Fair enough.
> Thanks for spending the time to look at this. Coaxing me to re-write the
> tail of do_machine_check() has made that code much better. Too many
> years of one patch on top of another without looking at the whole context.
> Cogitate on this series over the weekend and see if you can give me
> an Acked-by or Reviewed-by (I'll be adding a #define for BIT(63)).

I can't review the MCE decoding part, because I don't understand it
nearly well enough. The interaction with the core fault handling
looks fine, modulo any need to bikeshed on the macro naming (which
I'll refrain from doing).

I still think it would be better if you get rid of BIT(63) and use a
pair of landing pads, though. They could be as simple as:

xorq %rax, %rax
jmp .Lbad

/* set high bit of rax or whatever */
/* fall through */

/* deal with it */

That way the magic is isolated to the function that needs the magic.

Also, at least renaming the macro to EXTABLE_MC_PA_IN_AX might be
nice. It'll keep future users honest. Maybe some day there'll be a
PA_IN_AX flag, and, heck, maybe some day there'll be ways to get info
for non-MCE faults delivered through fixup_exception.

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