From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1D560C2D0DB for ; Wed, 29 Jan 2020 13:26:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EF27A214D8 for ; Wed, 29 Jan 2020 13:26:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726659AbgA2N0b (ORCPT ); Wed, 29 Jan 2020 08:26:31 -0500 Received: from mx2.suse.de ([195.135.220.15]:53592 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726140AbgA2N0a (ORCPT ); Wed, 29 Jan 2020 08:26:30 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id E459DAC46; Wed, 29 Jan 2020 13:26:28 +0000 (UTC) Date: Wed, 29 Jan 2020 14:26:18 +0100 From: Borislav Petkov To: Linus Torvalds Cc: Ingo Molnar , Tony Luck , Linux Kernel Mailing List , Thomas Gleixner , Borislav Petkov , Peter Zijlstra , Andrew Morton Subject: Re: [GIT PULL] x86/asm changes for v5.6 Message-ID: <20200129132618.GA30979@zn.tnic> References: <20200128165906.GA67781@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 28, 2020 at 12:06:53PM -0800, Linus Torvalds wrote: > On Tue, Jan 28, 2020 at 11:51 AM Linus Torvalds > wrote: > > > > ALTERNATIVE_2 \ > > "cmp $680, %rdx ; jb 3f ; cmpb %dil, %sil; je 4f", \ > > "movq %rdx, %rcx ; rep movsb; retq", X86_FEATURE_FSRM, \ > > "cmp $0x20, %rdx; jb 1f; movq %rdx, %rcx; rep movsb; retq", X86_FEATURE_ERMS > > Note the UNTESTED part. > > In particular, I didn't check what the priority for the alternatives > is. Since FSRM being set always implies ERMS being set too, it may be > that the ERMS case is always picked with the above code. > > So maybe the FSRM and ERMS lines need to be switched around, and > somebody should add a comment to the ALTERNATIVE_2 macro about the > priority rules for feature1 vs feature2 when both are set.. > > IOW, testing most definitely required for that patch suggestion of mine.. So what is there now before your patch is this (I've forced both X86_FEATURE_FSRM and X86_FEATURE_ERMS on a BDW guest). [ 4.238160] apply_alternatives: feat: 18*32+4, old: (__memmove+0x17/0x1a0 (ffffffff817d90d7) len: 10), repl: (ffffffff8251dbbb, len: 0), pad: 0 [ 4.239503] ffffffff817d90d7: old_insn: 48 83 fa 20 0f 82 f5 00 00 00 That's what in vmlinux: ffffffff817d90d7: 48 83 fa 20 cmp $0x20,%rdx ffffffff817d90db: 0f 82 f5 00 00 00 jb ffffffff817d91d6 which is 10 bytes. It gets replaced to: [ 4.240194] ffffffff817d90d7: final_insn: 0f 1f 84 00 00 00 00 00 66 90 ffffffff817d90d7: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) ffffffff817d90de: 00 ffffffff817d90df: 66 90 xchg %ax,%ax I.e., NOPed out. ERMS replaces the bytes *after* these 10 bytes, note the VA: 0xffffffff817d90d7 + 0xa = 0xffffffff817d90e1 [ 4.240917] apply_alternatives: feat: 9*32+9, old: (__memmove+0x21/0x1a0 (ffffffff817d90e1) len: 6), repl: (ffffffff8251dbbb, len: 6), pad: 6 [ 4.242209] ffffffff817d90e1: old_insn: 90 90 90 90 90 90 [ 4.242823] ffffffff8251dbbb: rpl_insn: 48 89 d1 f3 a4 c3 [ 4.243503] ffffffff817d90e1: final_insn: 48 89 d1 f3 a4 c3 which turns into ffffffff817d90e1: 48 89 d1 mov %rdx,%rcx ffffffff817d90e4: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi) ffffffff817d90e6: c3 retq as expected. And yes, your idea makes sense to use ALTERNATIVE_2 but as it is, it triple-faults my guest. I'll debug it more later to find out why, when I get a chance. -- Regards/Gruss, Boris. SUSE Software Solutions Germany GmbH, GF: Felix Imendörffer, HRB 36809, AG Nürnberg