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 17:52:54 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Tony Luck <tony.luck@...el.com>
Cc:	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 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?

--Andy