linux-kernel - Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options

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Date:	Fri, 8 Jan 2016 23:31:16 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Tony Luck <tony.luck@...el.com>, Ingo Molnar <mingo@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Robert <elliott@....com>,
	"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: [PATCH v8 1/3] x86: Expand exception table to allow new handling options

On Fri, Jan 8, 2016 at 10:39 PM, Brian Gerst <brgerst@...il.com> wrote:
> On Fri, Jan 8, 2016 at 8:52 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Fri, Jan 8, 2016 at 12:49 PM, Tony Luck <tony.luck@...el.com> wrote:
>>> Huge amounts of help from  Andy Lutomirski and Borislav Petkov to
>>> produce this. Andy provided the inspiration to add classes to the
>>> exception table with a clever bit-squeezing trick, Boris pointed
>>> out how much cleaner it would all be if we just had a new field.
>>>
>>> Linus Torvalds blessed the expansion with:
>>>   I'd rather not be clever in order to save just a tiny amount of space
>>>   in the exception table, which isn't really criticial for anybody.
>>>
>>> The third field is a simple integer indexing into an array of handler
>>> functions (I thought it couldn't be a relative pointer like the other
>>> fields because a module may have its ex_table loaded more than 2GB away
>>> from the handler function - but that may not be actually true. But the
>>> integer is pretty flexible, we are only really using low two bits now).
>>>
>>> We start out with three handlers:
>>>
>>> 0: Legacy - just jumps the to fixup IP
>>> 1: Fault - provide the trap number in %ax to the fixup code
>>> 2: Cleaned up legacy for the uaccess error hack
>>
>> I think I preferred the relative function pointer approach.
>>
>> Also, I think it would be nicer if the machine check code would invoke
>> the handler regardless of which handler (or class) is selected.  Then
>> the handlers that don't want to handle #MC can just reject them.
>>
>> Also, can you make the handlers return bool instead of int?
>
> I'm hashing up an idea that could eliminate alot of text in the .fixup
> section, but it needs the integer handler method to work.  We have
> alot of fixup code that does "mov $-EFAULT, reg; jmp xxxx".  If we
> encode the register in the third word, the handler can be generic and
> no fixup code for each user access would be needed.  That would
> recover alot of the memory used by expanding the exception table.

On second thought, this could still be implemented with a relative
function pointer.  We'd just need a separate function for each
register.

--
Brian Gerst