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

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Message-ID: <CALCETrUFQXPB9HM8O+4UfMij7nodfrWtjicy0XNhOiWCka+4yw@mail.gmail.com>
Date:	Fri, 11 Dec 2015 14:55:45 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	"Williams, Dan J" <dan.j.williams@...el.com>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>
Subject: Re: [PATCHV2 3/3] x86, ras: Add mcsafe_memcpy() function to recover
 from machine checks

On Fri, Dec 11, 2015 at 2:45 PM, Luck, Tony <tony.luck@...el.com> 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:

.Lpage_fault_goes_here:
    xorq %rax, %rax
    jmp .Lbad

.Lmce_goes_here:
    /* set high bit of rax or whatever */
    /* fall through */

.Lbad:
    /* 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.

--Andy
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