From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f42.google.com (mail-oi0-f42.google.com [209.85.218.42]) by kanga.kvack.org (Postfix) with ESMTP id 6FC95828DE for ; Fri, 8 Jan 2016 23:31:17 -0500 (EST) Received: by mail-oi0-f42.google.com with SMTP id k206so23184407oia.1 for ; Fri, 08 Jan 2016 20:31:17 -0800 (PST) Received: from mail-oi0-x244.google.com (mail-oi0-x244.google.com. [2607:f8b0:4003:c06::244]) by mx.google.com with ESMTPS id y196si17755523oie.8.2016.01.08.20.31.16 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 08 Jan 2016 20:31:16 -0800 (PST) Received: by mail-oi0-x244.google.com with SMTP id e195so14617189oig.2 for ; Fri, 08 Jan 2016 20:31:16 -0800 (PST) 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 Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org