Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932071AbZJVWRH (ORCPT ); Thu, 22 Oct 2009 18:17:07 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753311AbZJVWRG (ORCPT ); Thu, 22 Oct 2009 18:17:06 -0400 Received: from mail-ew0-f208.google.com ([209.85.219.208]:45448 "EHLO mail-ew0-f208.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752805AbZJVWRF (ORCPT ); Thu, 22 Oct 2009 18:17:05 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=googlemail.com; s=gamma; h=to:mail-followup-to:cc:subject:references:from:date:in-reply-to :message-id:user-agent:mime-version:content-type; b=h61SyMrOGBC4rTshUSUUg73Z5CaKDUI9pmZdhCsxUhGhcZlVK91mGAoAypivYKvTWQ E5O0bYUhY6GzTari4FtcMA6k9woANPHo/hdAbX66jHTdkPORIZwpu+SQB39Qo8KlR03w uK5jTzXJquP12ZFlZum4K8Ppf+fw+oJjUpjRo= To: David Daney Mail-Followup-To: David Daney ,wuzhangjin@gmail.com, Adam Nemet , rostedt@goodmis.org, linux-kernel@vger.kernel.org, linux-mips@linux-mips.org, Thomas Gleixner , Ralf Baechle , Nicholas Mc Guire , rdsandiford@googlemail.com Cc: wuzhangjin@gmail.com, Adam Nemet , rostedt@goodmis.org, linux-kernel@vger.kernel.org, linux-mips@linux-mips.org, Thomas Gleixner , Ralf Baechle , Nicholas Mc Guire Subject: Re: [PATCH -v4 4/9] tracing: add static function tracer support for MIPS References: <028867b99ec532b84963a35e7d552becc783cafc.1256135456.git.wuzhangjin@gmail.com> <2f73eae542c47ac5bbb9f7280e6c0271d193e90d.1256135456.git.wuzhangjin@gmail.com> <3f0d3515f74a58f4cfd11e61b62a129fdc21e3a7.1256135456.git.wuzhangjin@gmail.com> <1256138686.18347.3039.camel@gandalf.stny.rr.com> <1256233679.23653.7.camel@falcon> <4AE0A5BE.8000601@caviumnetworks.com> From: Richard Sandiford Date: Thu, 22 Oct 2009 23:17:06 +0100 In-Reply-To: <4AE0A5BE.8000601@caviumnetworks.com> (David Daney's message of "Thu\, 22 Oct 2009 11\:34\:38 -0700") Message-ID: <87y6n36plp.fsf@firetop.home> User-Agent: Gnus/5.11 (Gnus v5.11) Emacs/22.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2479 Lines: 62 David Daney writes: > Wu Zhangjin wrote: >> On Wed, 2009-10-21 at 11:24 -0400, Steven Rostedt wrote: > [...] >>>> + >>>> +NESTED(_mcount, PT_SIZE, ra) >>>> + RESTORE_SP_FOR_32BIT >>>> + PTR_LA t0, ftrace_stub >>>> + PTR_L t1, ftrace_trace_function /* please don't use t1 later, safe? */ >>> Is t0 and t1 safe for mcount to use? Remember, mcount does not follow >>> the dynamics of C function ABI. >> >> So, perhaps we can use the saved registers(a0,a1...) instead. >> > > a0..a7 may not always be saved. > > You can use at, v0, v1 and all the temporary registers. Note that for > the 64-bit ABIs sometimes the names t0-t4 shadow a4-a7. So for a 64-bit > kernel, you can use: $1, $2, $3, $12, $13, $14, $15, $24, $25, noting > that at == $1 and contains the callers ra. For a 32-bit kernel you can > add $8, $9, $10, and $11 > > This whole thing seems a little fragile. > > I think it might be a good idea to get input from Richard Sandiford, > and/or Adam Nemet about this approach (so I add them to the CC). > > This e-mail thread starts here: > > http://www.linux-mips.org/archives/linux-mips/2009-10/msg00286.html > > and here: > > http://www.linux-mips.org/archives/linux-mips/2009-10/msg00290.html I'm not sure that the "search for a save of RA" thing is really a good idea. The last version of that seemed to be "assume that any register stores will be in a block that immediately precedes the move into RA", but even if that's true now, it might not be in future. And as Wu Zhangjin says, it doesn't cope with long calls, where the target address is loaded into a temporary register before the call. FWIW, I'd certainly be happy to make GCC pass an additional parameter to _mcount. The parameter could give the address of the return slot, or null for leaf functions. In almost all cases[*], there would be no overhead, since the move would go in the delay slot of the call. [*] Meaning when the frame is <=32k. ;) I'm guessing you never get anywhere near that, and if you did, the scan thing wouldn't work anyway. The new behaviour could be controlled by a command-line option, which would also give linux a cheap way of checking whether the feature is available. Richard -- 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/