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

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Message-ID: <CALCETrU3OCVJoBWXcdmy-9Rr3d3rJ93606K1vC3V9zfT2bQc2g@mail.gmail.com>
Date:	Sat, 26 Dec 2015 18:16:59 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Tony Luck <tony.luck@...il.com>
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 Sat, Dec 26, 2015 at 6:15 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Sat, Dec 26, 2015 at 6:08 PM, Tony Luck <tony.luck@...il.com> wrote:
>> On Sat, Dec 26, 2015 at 6:54 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Dec 26, 2015 6:33 PM, "Borislav Petkov" <bp@...en8.de> wrote:
>>>> Andy, why is that? It makes the exception handling much simpler this way...
>>>>
>>>
>>> I like the idea of moving more logic into C, but I don't like
>>> splitting the logic across files and adding nasty special cases like
>>> this.
>>>
>>> But what if we generalized it?  An extable entry gives a fault IP and
>>> a landing pad IP.  Surely we can squeeze a flag bit in there.
>>
>> The clever squeezers have already been here. Instead of a pair
>> of 64-bit values for fault_ip and fixup_ip they managed with a pair
>> of 32-bit values that are each the relative offset of the desired address
>> from the table location itself.
>>
>> We could make one of them 31-bits (since even an "allyesconfig" kernel
>> is still much smaller than a gigabyte) to free a bit for a flag. But there
>> are those external tools to pre-sort exception tables that would all
>> need to be fixed too.

Wait, why?  The external tools sort by source address, and we'd
squeeze the flag into the target address, no?

--Andy
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