linux-kernel - Re: [PATCHV5 3/3] x86, ras: Add __mcsafe_copy() function to recover from machine checks

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Message-ID: <CA+8MBb+MaZUK1vMPNwUchZJed0Fi3vh9_vFP2OoPZsUMoDO=ZQ@mail.gmail.com>
Date:	Thu, 31 Dec 2015 12:30:42 -0800
From:	Tony Luck <tony.luck@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Borislav Petkov <bp@...en8.de>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86 ML <x86@...nel.org>,
	"elliott@....com" <elliott@....com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHV5 3/3] x86, ras: Add __mcsafe_copy() function to recover
 from machine checks

On Wed, Dec 30, 2015 at 3:32 PM, Tony Luck <tony.luck@...il.com> wrote:
> Fifth is just a hack because I clearly didn't understand what I was
> doing in parts 2&3 because my new class shows up as '3' not '1'!
>
> Andy: Can you explain the assembler/linker arithmetic for the class?

Never mind ... figured it out.

The fixup entry in the extable is:

label - . + 0x2000000 - BIAS

The "label - ." part evaluates to a smallish negative value (because
the .fixup section is bundled in towards the end of .text, and the
ex_table section comes right after.

Then you add 0x20000000 to get a positive number, then *subtract*
the BIAS.  I'd picked BIAS = 0x40000000 thinking that would show
up directly in class bits. But 0x1ffff000 - 0x40000000 is 0xdffff000
so bits 31 & 31 are both set, and this is class3

I switched to BIAS 0xC0000000 ... and now I get class 1 entries
(bit31=0, bit30=1).

New patch series coming soon.

-Tony
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