Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752639AbbDAPuI (ORCPT ); Wed, 1 Apr 2015 11:50:08 -0400 Received: from mail-ig0-f181.google.com ([209.85.213.181]:35543 "EHLO mail-ig0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751158AbbDAPuD (ORCPT ); Wed, 1 Apr 2015 11:50:03 -0400 MIME-Version: 1.0 In-Reply-To: <551BD241.4060207@redhat.com> References: <1427821211-25099-1-git-send-email-dvlasenk@redhat.com> <1427821211-25099-7-git-send-email-dvlasenk@redhat.com> <551BD241.4060207@redhat.com> Date: Wed, 1 Apr 2015 08:50:02 -0700 X-Google-Sender-Auth: b8uJ_xGeg4sFatkaz_tPm_wJiTw Message-ID: Subject: Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions From: Linus Torvalds To: Denys Vlasenko Cc: Brian Gerst , Ingo Molnar , Steven Rostedt , Borislav Petkov , "H. Peter Anvin" , Andy Lutomirski , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , "the arch/x86 maintainers" , Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1023 Lines: 23 On Wed, Apr 1, 2015 at 4:10 AM, Denys Vlasenko wrote: > > I did not know that. I was sure they are always zero extended. On all half-way modern cpu's they are. But on some older cpu's (possibly just the original 386) the segment move instructions basically are always 16-bit, and the operand size is ignored (so the 32-bit version is just smaller and faster to decode, because it doesn't have a 16-bit operand size prefix) Iirc, the same is true for the values pushed to memory on exceptions, so the 'cs/ss' values on the exception stack may not be reliable in the upper 16 bits. I don't remember if the same might be true of "pushl %Sseg". The intel architecture manual says segment registers are zero-extended on push. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/