Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933179AbXK2TR7 (ORCPT ); Thu, 29 Nov 2007 14:17:59 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1761678AbXK2TRw (ORCPT ); Thu, 29 Nov 2007 14:17:52 -0500 Received: from terminus.zytor.com ([198.137.202.10]:39389 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759973AbXK2TRv (ORCPT ); Thu, 29 Nov 2007 14:17:51 -0500 Message-ID: <474F1027.2020801@zytor.com> Date: Thu, 29 Nov 2007 11:16:55 -0800 From: "H. Peter Anvin" User-Agent: Thunderbird 2.0.0.9 (X11/20071115) MIME-Version: 1.0 To: Linus Torvalds , Andi Kleen CC: Chuck Ebbert , Roland McGrath , Andrew Morton , linux-kernel@vger.kernel.org, Thomas Gleixner , Ingo Molnar Subject: Re: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task References: <20071129003849.428E026F8E7@magilla.localdomain> <20071129004222.E49AD26F8E7@magilla.localdomain> <474EF824.3020806@redhat.com> <474F01F6.2030509@zytor.com> <474F08E1.2090806@zytor.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1815 Lines: 40 Andi, do you happen to remember the details on this? -hpa Linus Torvalds wrote: > > On Thu, 29 Nov 2007, H. Peter Anvin wrote: >> Linus Torvalds wrote: >>>> It is advantageous for user space to use the register the kernel typically >>>> won't, in order to speed up system call entry/exit. >>> but I'm not seeing the reason for that one. Care to comment more? (Yes, >>> there is often a latency from segment reload to use, but the reload latency >>> for system call exit *should* be entirely covered by the cost of doing the >>> system call return itself, no?) >> I do seem to recall that some processor implementations can load a NULL >> segment faster than a non-NULL segment. This was significant enough that we >> wanted to use %fs in x86-64 userspace, as opposed to the original ABI which >> used %gs both in userspace and in the kernel. > > Ahh, I think you may be right for some CPUs. The zero selector is indeed > potentially faster to load, since it doesn't have to even bother looking > at the GDT/LDT. > > That said, I doubt it's very noticeable. I just ran tests on both an old > P4 and on a more modern Core 2 machine, and for both of those the > performance was identical between loading a NUL selector and loading it > with a non-zero one. > > But I could well imagine that it matters a few cycles on other CPU's. But > from my testing, it definitely isn't noticeable, and I think the > maintenance advantage of using the same segment setup would more than make > up for the fact that maybe some odd CPU can see a difference. > > 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/