From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752847AbdK2Usn (ORCPT <rfc822;w@1wt.eu>);
        Wed, 29 Nov 2017 15:48:43 -0500
Received: from mail-io0-f196.google.com ([209.85.223.196]:35536 "EHLO
        mail-io0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751980AbdK2Usk (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 29 Nov 2017 15:48:40 -0500
X-Google-Smtp-Source: AGs4zMb7OBX0lMaCG1CteGZVFVlLtQT1hqMfGPAbvxJSD1Ehd7zJCViRf8+KqmNlVG0RP3MO8ZNVZgMq9I2s2/Dl3Uc=
MIME-Version: 1.0
In-Reply-To: <c3d2884bce244b0fa286cc5d1c07e23e@AcuMS.aculab.com>
References: <0fede9f9-88b0-a6e7-1027-dfb2019b8ef2@linux.intel.com>
 <20171129070951.hjjjpbyilzaak4ig@gmail.com> <20171129124711.slmq5emx4f26c4zp@hirez.programming.kicks-ass.net>
 <D6A6B532-57B5-4393-A322-DD441BEE74C4@amacapital.net> <4f2c1cf45fd04e83a7d360d0d602830e@AcuMS.aculab.com>
 <CALCETrWndxkJebKbxdCUVpa0_CkW9QdaMbE-d5M=XEw_JFG3YQ@mail.gmail.com> <c3d2884bce244b0fa286cc5d1c07e23e@AcuMS.aculab.com>
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 29 Nov 2017 12:48:39 -0800
X-Google-Sender-Auth: IT8gudG4wcMplrZuU3BjZed8KeY
Message-ID: <CA+55aFwYUDG01tYvpUFYz9RV2U9VREcso4Cdjtw63oJSYmq0QQ@mail.gmail.com>
Subject: Re: [PATCH] x86/entry/64: Fix native_load_gs_index() SWAPGS handling
 with IRQ state tracing enabled
To: David Laight <David.Laight@aculab.com>
Cc: Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Jarkko Nikula <jarkko.nikula@linux.intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Nov 29, 2017 at 8:51 AM, David Laight <David.Laight@aculab.com> wrote:
>
> SWAPGS is a PITA. The hardware designers should have tried to write
> all the interrupt handling code.

That's actually my biggest beef with x86-64. The kernel entry/exit is
completely misdesigned.

That was actually better in the original i386, which handles nesting
ok (modulo the NMI disable flag and the STI shadow bit which doesn't
save/restore properly).

Yes, the 386 model of infinite indirection through IDT/GDT/TSS is
broken and should have had a mode bit to just replace with a simple
"use this stack and address for kernel entry" MSR register - but that
simplified kernel entry/exit should have saved/restored _more_ info,
not less (eg "save/restore segment shadow state rather than the
descriptor number that needs the insane indirection" etc)

The problem was never the few push/pop instructions that kernel entry involved.

The entry/exit garbage admittedly started before x86-64 itself - all
those 'syscall' variants are equally broken. Not saving/restoring
state properly is just unbelievable sh*t. x86-64 then made things
worse with SWAPGS etc.

A lot of people hate x86 because of instruction decoding. No, if you
need a reason to dislike x86, it's because of exception handling and
iret.

Rant over.

                Linus