Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754763AbcKOTZQ (ORCPT ); Tue, 15 Nov 2016 14:25:16 -0500 Received: from mail-it0-f48.google.com ([209.85.214.48]:36285 "EHLO mail-it0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751496AbcKOTZO (ORCPT ); Tue, 15 Nov 2016 14:25:14 -0500 MIME-Version: 1.0 In-Reply-To: <20161115191807.GC25626@codeaurora.org> References: <20161018234200.5804-1-sboyd@codeaurora.org> <20161115191807.GC25626@codeaurora.org> From: Ard Biesheuvel Date: Tue, 15 Nov 2016 19:25:12 +0000 Message-ID: Subject: Re: [PATCH/RESEND] recordmcount: arm: Implement make_nop To: Stephen Boyd Cc: "Steven Rostedt (Red Hat)" , Andrew Morton , Russell King , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" 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: 2168 Lines: 55 On 15 November 2016 at 19:18, Stephen Boyd wrote: > On 11/15, Ard Biesheuvel wrote: >> On 19 October 2016 at 00:42, Stephen Boyd wrote: >> > In similar spirit to x86 and arm64 support, add a make_nop_arm() >> > to replace calls to mcount with a nop in sections that aren't >> > traced. >> > >> > Cc: Russell King >> > Acked-by: Rabin Vincent >> > Signed-off-by: Stephen Boyd >> > --- >> > scripts/recordmcount.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++ >> > 1 file changed, 65 insertions(+) >> > >> > diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c >> > index 5423a58d1b06..aeb34223167c 100644 >> > --- a/scripts/recordmcount.c >> > +++ b/scripts/recordmcount.c >> > @@ -213,6 +213,59 @@ static int make_nop_x86(void *map, size_t const offset) >> > return 0; >> > } >> > >> > +static unsigned char ideal_nop4_arm_le[4] = { 0x00, 0x00, 0xa0, 0xe1 }; /* mov r0, r0 */ >> > +static unsigned char ideal_nop4_arm_be[4] = { 0xe1, 0xa0, 0x00, 0x00 }; /* mov r0, r0 */ >> >> Shouldn't you be taking the difference between BE8 and BE32 into >> account here? IIRC, BE8 uses little endian encoding for instructions. > > I admit I haven't tested on a pre-armv6 CPU so I haven't come > across the case of a BE32 CPU. But from what I can tell that > doesn't matter. > > According to scripts/Makefile.build, cmd_record_mcount only runs > the recordmcount program if CONFIG_FTRACE_MCOUNT_RECORD=y. That > config is defined as: > > config FTRACE_MCOUNT_RECORD > def_bool y > depends on DYNAMIC_FTRACE > depends on HAVE_FTRACE_MCOUNT_RECORD > > > And in arch/arm/Kconfig we see that DYNAMIC_FTRACE is selected: > > select HAVE_DYNAMIC_FTRACE if (!XIP_KERNEL) && !CPU_ENDIAN_BE32 && MMU > > which means that FTRACE_MCOUNT_RECORD can't be set when > CPU_ENDIAN_BE32 is set. > > Do you agree that BE32 is not a concern here? > Yes. But that implies then that you should not be using big-endian instruction encodings at all, and simply use the _le variants for both LE and BE8