From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754622AbeBGU1L (ORCPT ); Wed, 7 Feb 2018 15:27:11 -0500 Received: from terminus.zytor.com ([65.50.211.136]:53603 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754397AbeBGU1J (ORCPT ); Wed, 7 Feb 2018 15:27:09 -0500 Subject: Re: [RFC 0/3] x86: Patchable constants To: Linus Torvalds , "Kirill A. Shutemov" Cc: the arch/x86 maintainers , Tom Lendacky , Peter Zijlstra , Dave Hansen , Andy Lutomirski , Borislav Petkov , linux-mm , Linux Kernel Mailing List References: <20180207145913.2703-1-kirill.shutemov@linux.intel.com> From: "H. Peter Anvin" Message-ID: <8fea57cc-8772-f8b4-3298-91b0de126358@zytor.com> Date: Wed, 7 Feb 2018 12:20:09 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/07/18 09:01, Linus Torvalds wrote: > > Look - much smaller code, and register %rcx isn't used at all. And no > D$ miss on loading that constant (that is a constant depending on > boot-time setup only). > > It's rather more complex, but it actually gives a much bigger win. The > code itself will be much better, and smaller. > > The *infrastructure* for the code gets pretty hairy, though. > > The good news is that the patch already existed to at least _some_ > degree. Peter Anvin did it about 18 months ago. > > It was not really pursued all the way because it *is* a lot of extra > complexity, and I think there was some other hold-up, but he did have > skeleton code for the actual replacement. > > There was a thread on the x86 arch list with the subject line > > Disgusting pseudo-self-modifying code idea: "variable constants" > > but I'm unable to actually find the patch. I know there was at least a > vert early prototype. > > Adding hpa to the cc in the hope that he has some prototype code still > laying around.. > The patchset I have is about 85% complete. It mostly needs cleanup, testing, and breaking into reasonable chunks (it got put on the backburner for somewhat obvious reasons, but I don't think it'll take very long at all to productize it.) The main reason I haven't submitted it yet is that I got a bit overly ambitious and wanted to implement a whole bunch of more complex subcases, such as 64-bit shifts on a 32-bit kernel. The win in that case is actually quite huge, but it is requires data-dependent code patching and not just immediate patching, which requires augmentation of the alternatives framework. -hpa