Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754206AbbGUMHE (ORCPT ); Tue, 21 Jul 2015 08:07:04 -0400 Received: from mx1.redhat.com ([209.132.183.28]:47903 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752345AbbGUMG7 (ORCPT ); Tue, 21 Jul 2015 08:06:59 -0400 Date: Tue, 21 Jul 2015 07:06:57 -0500 From: Josh Poimboeuf To: Ingo Molnar Cc: Andy Lutomirski , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Michal Marek , Peter Zijlstra , Andy Lutomirski , Borislav Petkov , Linus Torvalds , Andi Kleen , Pedro Alves , X86 ML , live-patching@vger.kernel.org, "linux-kernel@vger.kernel.org" Subject: Re: [RFC PATCH 13/21] x86/asm/crypto: Fix frame pointer usage in aesni-intel_asm.S Message-ID: <20150721120657.GC11122@treble.redhat.com> References: <2ea0f0602978178eafd012e52b8bdb83cfb159d5.1437150175.git.jpoimboe@redhat.com> <20150717194307.GA26757@gmail.com> <20150717203746.GB12761@treble.redhat.com> <20150718025116.GB13059@gmail.com> <20150718035623.GA22664@treble.redhat.com> <20150720163646.GA28075@treble.redhat.com> <20150721080016.GA26811@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20150721080016.GA26811@gmail.com> User-Agent: Mutt/1.5.23.1-rc1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1943 Lines: 45 On Tue, Jul 21, 2015 at 10:00:16AM +0200, Ingo Molnar wrote: > > * Josh Poimboeuf wrote: > > > On Mon, Jul 20, 2015 at 08:30:52AM -0700, Andy Lutomirski wrote: > > > On Fri, Jul 17, 2015 at 8:56 PM, Josh Poimboeuf wrote: > > > > The reason I suggested to put FRAME in the macro name is to try to > > > > prevent it from being accidentally used for leaf functions, where it > > > > isn't needed. > > > > > > > > > > Could someone remind me why it isn't needed for leaf functions? > > > > If a function doesn't call any other functions, then it won't ever show > > up in a stack trace unless: > > > > a) the function itself walks the stack, in which case the frame pointer > > isn't necessary; or > > > > b) The function gets hit by an interrupt/exception, in which case frame > > pointers can't be 100% relied upon anyway. > > > > I've noticed that gcc *does* seem to create stack frames for leaf functions. > > But it's inconsistent, because the early exit path of some functions will skip > > the stack frame creation and go straight to the return. > > > > We could probably get a good performance boost with the > > -momit-leaf-frame-pointer flag. Though it would make stack traces less reliable > > when a leaf function gets interrupted. > > So in theory we could resolve this during the stack walk: when we pass from the > IRQ stack to the process stack we actually know the RIP of the interrupted > context, and could include that. The problem is with the *caller* of the leaf function. Without the leaf's frame pointer there's no way to find the call site pointer on the stack, so the leaf's caller gets skipped. -- Josh -- 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/