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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 5D632C10F29 for ; Tue, 17 Mar 2020 09:16:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 40A71205ED for ; Tue, 17 Mar 2020 09:16:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725730AbgCQJQV (ORCPT ); Tue, 17 Mar 2020 05:16:21 -0400 Received: from mx2.suse.de ([195.135.220.15]:55206 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725862AbgCQJQV (ORCPT ); Tue, 17 Mar 2020 05:16:21 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 69A5BAC67; Tue, 17 Mar 2020 09:16:19 +0000 (UTC) Date: Tue, 17 Mar 2020 10:16:18 +0100 (CET) From: Miroslav Benes To: Josh Poimboeuf cc: =?ISO-8859-15?Q?J=FCrgen_Gro=DF?= , boris.ostrovsky@oracle.com, sstabellini@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com, x86@kernel.org, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, live-patching@vger.kernel.org, jslaby@suse.cz, Peter Zijlstra Subject: Re: [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks reliable In-Reply-To: <20200316203514.qm7so7b55jbmskgg@treble> Message-ID: References: <20200312142007.11488-1-mbenes@suse.cz> <20200312142007.11488-3-mbenes@suse.cz> <75224ad1-f160-802a-9d72-b092ba864fb7@suse.com> <20200316203514.qm7so7b55jbmskgg@treble> User-Agent: Alpine 2.21 (LSU 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: live-patching-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org On Mon, 16 Mar 2020, Josh Poimboeuf wrote: > On Mon, Mar 16, 2020 at 04:51:12PM +0100, Miroslav Benes wrote: > > > diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c > > > index 6b88cdcbef8f..39afd88309cb 100644 > > > --- a/arch/x86/xen/smp_pv.c > > > +++ b/arch/x86/xen/smp_pv.c > > > @@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void) > > > { > > > cpu_bringup(); > > > boot_init_stack_canary(); > > > + asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1)); > > > cpu_startup_entry(CPUHP_AP_ONLINE_IDLE); > > > } > > > > > > and that seems to work. I need to properly verify and test, but the > > > explanation is that as opposed to the above, cpu_startup_entry() is on the > > > idle task's stack and the hint is then taken into account. The unwound > > > stack seems to be complete, so it could indeed be the fix. > > > > Not the correct one though. Objtool rightfully complains with > > > > arch/x86/xen/smp_pv.o: warning: objtool: cpu_bringup_and_idle()+0x6a: undefined stack state > > > > and all the other hacks I tried ended up in the same dead alley. It seems > > to me the correct fix is that all orc entries for cpu_bringup_and_idle() > > should have "end" property set to 1, since it is the first function on the > > stack. I don't know how to achieve that without the assembly hack in the > > patch I sent. If I am not missing something, of course. > > > > Josh, any idea? > > Yeah, I think mucking with the unwind hints in C code is going to be > precarious. You could maybe have something like > > asm(" > UNWIND_HINT_EMPTY\n > mov $CPUHP_AP_ONLINE_IDLE, %rdi\n > call cpu_startup_entry\n > )" > unreachable(); > > but that's pretty ugly (and it might not work anyway). > > I suppose we could add a new facility to mark an entire C function as an > "end" point. I think it would be an overkill for what I perceive as one-off scenario. Maybe if there are more use cases in the future, but I doubt it. > But I think it would be cleanest to just do something like your patch > and have the entry code be asm which then calls cpu_bringup_and_idle(). > That would make it consistent with all other entry code, which all lives > in asm. Ack. Thanks Miroslav