From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754083AbbAPQWu (ORCPT ); Fri, 16 Jan 2015 11:22:50 -0500 Received: from foss-mx-na.foss.arm.com ([217.140.108.86]:36314 "EHLO foss-mx-na.foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751736AbbAPQWt (ORCPT ); Fri, 16 Jan 2015 11:22:49 -0500 Date: Fri, 16 Jan 2015 16:22:44 +0000 From: Will Deacon To: Pratyush Anand Cc: Catalin Marinas , Steve Capper , Oleg Nesterov , "linux-kernel@vger.kernel.org" , David Long , William Cohen , "linux-arm-kernel@lists.infradead.org" Subject: Re: Query: ARM64: Behavior of el1_dbg exception while executing el0_dbg Message-ID: <20150116162244.GA7091@arm.com> References: <20150108162312.GO11583@arm.com> <54AEBE45.9080203@redhat.com> <20150109154635.GG11258@arm.com> <54B00C39.1070809@redhat.com> <20150112173005.GK13360@arm.com> <54B4BF4C.2030009@redhat.com> <20150113155221.GG16524@e104818-lin.cambridge.arm.com> <54B55B95.1070402@redhat.com> <54B7EF3C.8060909@redhat.com> <54B8FD49.2060503@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <54B8FD49.2060503@redhat.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 16, 2015 at 12:00:09PM +0000, Pratyush Anand wrote: > On Thursday 15 January 2015 10:17 PM, Pratyush Anand wrote: > > On Tuesday 13 January 2015 11:23 PM, Pratyush Anand wrote: > >> I will still try to find some way to capture enable_dbg macro path.H > > > > I did instrumented debug tap points at all the location from where > > enable_debug macro is called(see attached debug patch). But, I do not > > see that, execution reaches to any of those tap points between el0_dbg > > and el1_dbg, and tap points debug log also confirms that el1_dbg is > > raised before el0_dbg is returned. > > Probably we all missed this, ARMv8 specs is very clear about it. In > section "D2.1 About debug exceptions" it says: > > Software Breakpoint Instruction exceptions cannot be masked. The PE > takes Software Breakpoint Instruction exceptions regardless of both of > the following: > • The current Exception level. > • The current Security state. Ah, of course, I completely forgot you were using software breakpoints! > So, reception of el1_dbg while executing el0_dbg seems perfectly normal > to me. If you agree then I am back with the original query which I asked > in the beginning of the > thread,(http://permalink.gmane.org/gmane.linux.ports.arm.kernel/383672) > ie how can instruction_pointer be wrong when second el1_dbg is called > recursively(as follows). > > [1]-> el0_dbg (After executing BRK instruction by user) > [2] -> el1_dbg (when uprobe break handler at [1] executes BRK instruction) > (At the end of this ELR_EL1 is programmed with fffffdfffc000004) > [3] -> el1_dbg (when kprobe break handler at [2] enables single stepping) > (Here ELR_EL1 was found fffffe0000092470).So When this el1_dbg was > received, then regs->pc values are not same what was programmed in > ELR_EL1 at the return of [2]. Perhaps you're not removing the BRK instruction properly, and so you try to single-step a trapping instruction and end up stepping into the exception? Will