From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752162AbcAIEbS (ORCPT ); Fri, 8 Jan 2016 23:31:18 -0500 Received: from mail-oi0-f66.google.com ([209.85.218.66]:35992 "EHLO mail-oi0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750791AbcAIEbR (ORCPT ); Fri, 8 Jan 2016 23:31:17 -0500 MIME-Version: 1.0 In-Reply-To: References: <3a259f1cce4a3c309c2f81df715f8c2c9bb80015.1452297867.git.tony.luck@intel.com> Date: Fri, 8 Jan 2016 23:31:16 -0500 Message-ID: Subject: Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options From: Brian Gerst To: Andy Lutomirski Cc: Tony Luck , Ingo Molnar , Borislav Petkov , Andrew Morton , Andy Lutomirski , Dan Williams , Robert , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , linux-nvdimm , X86 ML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 8, 2016 at 10:39 PM, Brian Gerst wrote: > On Fri, Jan 8, 2016 at 8:52 PM, Andy Lutomirski wrote: >> On Fri, Jan 8, 2016 at 12:49 PM, Tony Luck 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