Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031210AbbDWWjM (ORCPT ); Thu, 23 Apr 2015 18:39:12 -0400 Received: from mail-lb0-f181.google.com ([209.85.217.181]:33554 "EHLO mail-lb0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1031124AbbDWWjJ (ORCPT ); Thu, 23 Apr 2015 18:39:09 -0400 MIME-Version: 1.0 In-Reply-To: <553972D5.2070104@zytor.com> References: <1429792491-5978-1-git-send-email-dvlasenk@redhat.com> <20150423211015.GS28327@pd.tnic> <55396617.4020303@zytor.com> <553972D5.2070104@zytor.com> From: Andy Lutomirski Date: Thu, 23 Apr 2015 15:38:47 -0700 Message-ID: Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary To: "H. Peter Anvin" Cc: Borislav Petkov , Denys Vlasenko , Linus Torvalds , Brian Gerst , Denys Vlasenko , Ingo Molnar , Steven Rostedt , 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: 1046 Lines: 26 On Thu, Apr 23, 2015 at 3:31 PM, H. Peter Anvin wrote: > On 04/23/2015 03:29 PM, Andy Lutomirski wrote: >>> >>> Yes, the NULL SS is a special thing in 64-bit mode. I agree that >>> context-switching it is probably the way to go; it should be cheap >>> enough. We might even be able to conditionalize it on an X86_BUG_ flag. >> >> I still don't see why context switches are a better place than just >> before sysret, but I could be convinced. >> > > Because there are way more sysrets than context switches, and Linux is > particularly sensitive to system call latency, by design. I mean sysret but only when SS might be zero. Denys' approach apparently needs ~4 cycles to check that (not bad), we could (yuck) set a ti flag on context switch. But yes, maybe you're right. --Andy -- 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/