From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751652AbeFEKWi (ORCPT ); Tue, 5 Jun 2018 06:22:38 -0400 Received: from mail.kernel.org ([198.145.29.99]:58256 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751442AbeFEKWh (ORCPT ); Tue, 5 Jun 2018 06:22:37 -0400 X-Google-Smtp-Source: ADUXVKKHwawTxKjP9wl/UW3JbONgSJHUSbFRywvR1lPplWXjrn2+dRFS0xkqkLbRP6sSnZqg9nnftLt+CJSzZsTBXsI= MIME-Version: 1.0 References: <1528140269-26205-1-git-send-email-chang.seok.bae@intel.com> <1528140269-26205-7-git-send-email-chang.seok.bae@intel.com> <8a41304a-3517-003a-badf-1ba8f7ababe4@nextfour.com> <51587908-9652-6a8e-0e01-f387e3ae5852@zytor.com> In-Reply-To: <51587908-9652-6a8e-0e01-f387e3ae5852@zytor.com> From: Andy Lutomirski Date: Tue, 5 Jun 2018 03:22:22 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH 6/6] x86/vdso: Move out the CPU number store To: "H. Peter Anvin" Cc: =?UTF-8?Q?Mika_Penttil=C3=A4?= , "Bae, Chang Seok" , Andrew Lutomirski , Thomas Gleixner , Ingo Molnar , Andi Kleen , Dave Hansen , "Metzger, Markus T" , "Ravi V. Shankar" , LKML 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-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id w55AMhrf000372 On Mon, Jun 4, 2018 at 10:36 PM H. Peter Anvin wrote: > > On 06/04/18 20:57, Mika Penttilä wrote: > > > > This won't work on X86-32 because it actually uses the segment limit with fs: access. So there > > is a reason why the lsl based method is X86-64 only. > > > > > > Why does that matter in any shape, way, or form? The LSL instruction > doesn't touch any of the segment registers, it just uses a segment > selector number. > > > > I see... we have a VERY unfortunate name collision: the x86-64 > GDT_ENTRY_PERC_PU and the i386 GDT_ENTRY_PERCPU are in fact two > completely different things, with the latter being the actual percpu > offset used by the kernel. > > So yes, this patch is wrong, because the naming of the x86-64 segment is > insane especially in the proximity of the -- it should be something > like GDT_ENTRY_CPU_NUMBER. > > Unfortunately we probably can't use the same GDT entry on x86-32 and > x86-64, because entry 15 (selector 0x7b) is USER_DS, which is something > we really don't want to screw with. This means i386 programs that > execute LSL directly for whatever reason will have to understand the > difference, but most of the other segment numbers differ as well, > including user space %cs (USER_CS/USER32_CS) and %ds/%es/%ss (USER_DS). > Perhaps we could bump down segments 23-28 by one and put it as 23, that > way the CPU_NUMBER segment would always be at %ss+80 for the default > (flat, initial) user space %ss. (We want to specify using %ss rather > than %ds, because it is less likely to be changed and because 64 bits, > too, have %ss defined, but not %ds.) > > > > Rename the x86-64 segment to CPU_NUMBER, fixing the naming conflict. Yes, agreed. Probably as its own patch *before* the rest of this cleanup. > Add 1 to GDT entry numbers 23-28 for i386 (all of these are > kernel-internal segments and so have no impact on user space). > Add i386 CPU_NUMBER equivalent to x86-64 at GDT entry 23. > Document the above relationship between segments. Sure, but also as a standalone patch, please. > > OK, everyone? > > -hpa