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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 12121C0044D for ; Mon, 16 Mar 2020 15:51:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E8E8E20719 for ; Mon, 16 Mar 2020 15:51:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732100AbgCPPvP (ORCPT ); Mon, 16 Mar 2020 11:51:15 -0400 Received: from mx2.suse.de ([195.135.220.15]:39584 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731934AbgCPPvP (ORCPT ); Mon, 16 Mar 2020 11:51:15 -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 3F8AAAD5F; Mon, 16 Mar 2020 15:51:13 +0000 (UTC) Date: Mon, 16 Mar 2020 16:51:12 +0100 (CET) From: Miroslav Benes To: jpoimboe@redhat.com, =?ISO-8859-15?Q?J=FCrgen_Gro=DF?= cc: 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 Subject: Re: [RFC PATCH 2/2] x86/xen: Make the secondary CPU idle tasks reliable In-Reply-To: Message-ID: References: <20200312142007.11488-1-mbenes@suse.cz> <20200312142007.11488-3-mbenes@suse.cz> <75224ad1-f160-802a-9d72-b092ba864fb7@suse.com> User-Agent: Alpine 2.21 (LSU 202 2017-01-01) MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="1678380546-80487206-1584373873=:15518" Sender: live-patching-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: live-patching@vger.kernel.org This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --1678380546-80487206-1584373873=:15518 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT On Fri, 13 Mar 2020, Miroslav Benes wrote: > On Fri, 13 Mar 2020, Jürgen Groß wrote: > > > On 12.03.20 15:20, Miroslav Benes wrote: > > > The unwinder reports the secondary CPU idle tasks' stack on XEN PV as > > > unreliable, which affects at least live patching. > > > cpu_initialize_context() sets up the context of the CPU through > > > VCPUOP_initialise hypercall. After it is woken up, the idle task starts > > > in cpu_bringup_and_idle() function and its stack starts at the offset > > > right below pt_regs. The unwinder correctly detects the end of stack > > > there but it is confused by NULL return address in the last frame. > > > > > > RFC: I haven't found the way to teach the unwinder about the state of > > > the stack there. Thus the ugly hack using assembly. Similar to what > > > startup_xen() has got for boot CPU. > > > > > > It introduces objtool "unreachable instruction" warning just right after > > > the jump to cpu_bringup_and_idle(). It should show the idea what needs > > > to be done though, I think. Ideas welcome. > > > > > > Signed-off-by: Miroslav Benes > > > --- > > > arch/x86/xen/smp_pv.c | 3 ++- > > > arch/x86/xen/xen-head.S | 10 ++++++++++ > > > 2 files changed, 12 insertions(+), 1 deletion(-) > > > > > > diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c > > > index 802ee5bba66c..6b88cdcbef8f 100644 > > > --- a/arch/x86/xen/smp_pv.c > > > +++ b/arch/x86/xen/smp_pv.c > > > @@ -53,6 +53,7 @@ static DEFINE_PER_CPU(struct xen_common_irq, xen_irq_work) > > > = { .irq = -1 }; > > > static DEFINE_PER_CPU(struct xen_common_irq, xen_pmu_irq) = { .irq = -1 }; > > > > > > static irqreturn_t xen_irq_work_interrupt(int irq, void *dev_id); > > > +extern unsigned char asm_cpu_bringup_and_idle[]; > > > > > > static void cpu_bringup(void) > > > { > > > > Would adding this here work? > > > > + asm volatile (UNWIND_HINT(ORC_REG_UNDEFINED, 0, ORC_TYPE_CALL, 1)); > > I tried something similar. It did not work, because than the hint is > "bound" to the closest next call in the function which is cr4_init() in > this case. The unwinder would not take it into account. > > In my case, I placed it at the beginning of cpu_bringup_and_idle(). I also > open coded it and played with the offset in the orc entry, but that did > not work for some other reason. > > However, now I tried this > > 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? Thanks Miroslav --1678380546-80487206-1584373873=:15518--