Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751860AbdIUMSi (ORCPT ); Thu, 21 Sep 2017 08:18:38 -0400 Received: from mail-qt0-f175.google.com ([209.85.216.175]:51192 "EHLO mail-qt0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751709AbdIUMSg (ORCPT ); Thu, 21 Sep 2017 08:18:36 -0400 X-Google-Smtp-Source: AOwi7QDY4ui1HOpMHhJ/h/XahXR84sF7eypbBWWlpzDu9FA4z6loZcH80mbz+frETDmdjIXdBQiYUIXDCS3y842/V2A= MIME-Version: 1.0 In-Reply-To: References: <31e96e6bcfcb47725e15a093b9c31660dfaad430.1505846562.git.jpoimboe@redhat.com> <7e39ef18-3e60-8cc9-ec4f-1cd02ade171f@zytor.com> <20170920210731.kbcibdmbd4b3ppfi@treble> From: Dmitry Vyukov Date: Thu, 21 Sep 2017 14:18:13 +0200 Message-ID: Subject: Re: [PATCH 2/2] x86/asm: Fix inline asm call constraints for clang To: Brian Gerst Cc: Andy Lutomirski , Josh Poimboeuf , "H. Peter Anvin" , "x86@kernel.org" , LKML , Ingo Molnar , Thomas Gleixner , Andy Lutomirski , Linus Torvalds , Alexander Potapenko , Matthias Kaehlcke , Arnd Bergmann , Peter Zijlstra , Andrey Ryabinin 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: 2386 Lines: 51 On Thu, Sep 21, 2017 at 1:52 PM, Brian Gerst wrote: >>>>>>> I think we need just the frame itself and RSP pointing below this >>>>>>> frame. If we don't have a frame, CALL instruction will smash whatever >>>>>>> RSP happens to point to. Compiler doesn't have to setup RSP to point >>>>>>> below used part of stack in leaf functions. >>>>>>> >>>>>> >>>>>> In the kernel it does. Redzoning is not allowed in the kernel, because >>>>>> interrupts or exceptions would also smash the redzone. >>>>> >>>>> I see... But it's the same for user-space signals, the first thing a >>>>> signal should do is to skip the redzone. I guess interrupt handlers >>>>> should switch to interrupt stack which avoids smashing redzone >>>>> altogether. Do you mean nested interrupts/exceptions in interrupts? >>>>> In my experience frames in leaf functions can have pretty large >>>>> performance penalty. Wonder if we have we considered changing >>>>> interrupt/exception handlers to avoid smashing redzones and disable >>>>> leaf frames? >>>> >>>> Currently, on x86-64, I believe all exceptions have their own dedicated >>>> stacks in the kernel, but IRQs still come in on the task's kernel stack. >>>> >>>> Andy, do you know if there's a reason why IRQs don't use a dedicated IST >>>> stack? >>>> >>> >>> Because IST is awful due to recursion issues. We immediately switch to an IRQ stack, though. >>> >>> If the kernel wanted a redzone, it would have to use IST for everything, which would entail a bunch of unpleasant hackery. >> >> Thanks. >> >> I guess it must be finite recursion, because we could not handle >> infinite with finite stack. I thing that solves it is simply: >> >> sub $256, %rsp >> ... do stuff ... >> add $256, %rsp >> >> Don't know if it's applicable to interrupts or not. > > No, it is not. The processor pushes 5 or 6 words of data on the stack > (the IRET frame plus an error code for certain exceptions) before the > interrupt handler gets control. So without using the IST for stack > switching on every interrupt, the redzone cannot be used in the kernel > as it will get smashed by the IRET frame. In addition, since the > kernel's stack is limited in size, skipping 128 bytes on every > interrupt would overrun the stack faster. The small gain from using > the redzone in the kernel is outweighed by these limitations. I see, thanks for educating.