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

From: Tony Luck <tony.luck@gmail.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>,
	linux-nvdimm <linux-nvdimm@ml01.01.org>, X86 ML <x86@kernel.org>,
	"elliott@hpe.com" <elliott@hpe.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Williams, Dan J" <dan.j.williams@intel.com>,
	Ingo Molnar <mingo@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCHV5 3/3] x86, ras: Add __mcsafe_copy() function to recover from machine checks
Date: Thu, 31 Dec 2015 12:30:42 -0800	[thread overview]
Message-ID: <CA+8MBb+MaZUK1vMPNwUchZJed0Fi3vh9_vFP2OoPZsUMoDO=ZQ@mail.gmail.com> (raw)
In-Reply-To: <CA+8MBbK842Ov74ZSU_fmxoZNw_72J+3hg3KQ4C5aBjd_cDYfAA@mail.gmail.com>

On Wed, Dec 30, 2015 at 3:32 PM, Tony Luck <tony.luck@gmail.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