From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_NEOMUTT autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 56BB2C31E45 for ; Fri, 14 Jun 2019 01:58:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 334F52168B for ; Fri, 14 Jun 2019 01:58:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726581AbfFNB6w (ORCPT ); Thu, 13 Jun 2019 21:58:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42832 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726011AbfFNB6w (ORCPT ); Thu, 13 Jun 2019 21:58:52 -0400 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id A6CBB3083394; Fri, 14 Jun 2019 01:58:51 +0000 (UTC) Received: from treble (ovpn-121-232.rdu2.redhat.com [10.10.121.232]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 9918119C67; Fri, 14 Jun 2019 01:58:50 +0000 (UTC) Date: Thu, 13 Jun 2019 20:58:48 -0500 From: Josh Poimboeuf To: Alexei Starovoitov Cc: x86@kernel.org, linux-kernel@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , netdev@vger.kernel.org, bpf@vger.kernel.org, Peter Zijlstra , Song Liu , Kairui Song Subject: Re: [PATCH 7/9] x86/unwind/orc: Fall back to using frame pointers for generated code Message-ID: <20190614015848.todgfogryjn573nd@treble> References: <4f536ec4facda97406273a22a4c2677f7cb22148.1560431531.git.jpoimboe@redhat.com> <20190613220054.tmonrgfdeie2kl74@ast-mbp.dhcp.thefacebook.com> <20190614013051.6gnwduy4dsygbamj@treble> <20190614014244.st7fbr6areazmyrb@ast-mbp.dhcp.thefacebook.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190614014244.st7fbr6areazmyrb@ast-mbp.dhcp.thefacebook.com> User-Agent: NeoMutt/20180716 X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Fri, 14 Jun 2019 01:58:51 +0000 (UTC) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Thu, Jun 13, 2019 at 06:42:45PM -0700, Alexei Starovoitov wrote: > On Thu, Jun 13, 2019 at 08:30:51PM -0500, Josh Poimboeuf wrote: > > On Thu, Jun 13, 2019 at 03:00:55PM -0700, Alexei Starovoitov wrote: > > > > @@ -392,8 +402,16 @@ bool unwind_next_frame(struct unwind_state *state) > > > > * calls and calls to noreturn functions. > > > > */ > > > > orc = orc_find(state->signal ? state->ip : state->ip - 1); > > > > - if (!orc) > > > > - goto err; > > > > + if (!orc) { > > > > + /* > > > > + * As a fallback, try to assume this code uses a frame pointer. > > > > + * This is useful for generated code, like BPF, which ORC > > > > + * doesn't know about. This is just a guess, so the rest of > > > > + * the unwind is no longer considered reliable. > > > > + */ > > > > + orc = &orc_fp_entry; > > > > + state->error = true; > > > > > > That seems fragile. > > > > I don't think so. The unwinder has sanity checks to make sure it > > doesn't go off the rails. And it works just fine. The beauty is that > > it should work for all generated code (not just BPF). > > > > > Can't we populate orc_unwind tables after JIT ? > > > > As I mentioned it would introduce a lot more complexity. For each JIT > > function, BPF would have to tell ORC the following: > > > > - where the BPF function lives > > - how big the stack frame is > > - where RBP and other callee-saved regs are on the stack > > that sounds like straightforward addition that ORC should have anyway. > right now we're not using rbp in the jit-ed code, > but one day we definitely will. > Same goes for r12. It's reserved right now for 'strategic use'. > We've been thinking to add another register to bpf isa. > It will map to r12 on x86. arm64 and others have plenty of regs to use. > The programs are getting bigger and register spill/fill starting to > become a performance concern. Extra register will give us more room. With CONFIG_FRAME_POINTER, RBP isn't available. If you look at all the code in the entire kernel you'll notice that BPF JIT is pretty much the only one still clobbering it. -- Josh