From mboxrd@z Thu Jan 1 00:00:00 1970 From: panand@redhat.com (Pratyush Anand) Date: Tue, 13 Jan 2015 12:16:36 +0530 Subject: Query: ARM64: Behavior of el1_dbg exception while executing el0_dbg In-Reply-To: <20150112173005.GK13360@arm.com> References: <54AE830E.1090000@redhat.com> <20150108162312.GO11583@arm.com> <54AEBE45.9080203@redhat.com> <20150109154635.GG11258@arm.com> <54B00C39.1070809@redhat.com> <20150112173005.GK13360@arm.com> Message-ID: <54B4BF4C.2030009@redhat.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Monday 12 January 2015 11:00 PM, Will Deacon wrote: > On Fri, Jan 09, 2015 at 05:13:29PM +0000, Pratyush Anand wrote: >> >> >> On Friday 09 January 2015 09:16 PM, Will Deacon wrote: >>> On Thu, Jan 08, 2015 at 05:28:37PM +0000, Pratyush Anand wrote: >>>> On Thursday 08 January 2015 09:53 PM, Will Deacon wrote: >>>>> On Thu, Jan 08, 2015 at 01:15:58PM +0000, Pratyush Anand wrote: >>>>>> I am trying to test following scenario, which seems valid to me. But I >>>>>> am very new to ARM64 as well as to debugging tools, so seeking expert's >>>>>> comment here. >>>>>> >>>>>> -- I have inserted a kprobe to the function uprobe_breakpoint_handler >>>>>> which is called from elo_dbg >>>>>> (el0_dbg->do_debug_exception->brk_handler->call_break_hook->uprobe_breakpoint_handler) >>>>>> >>>>>> -- kprobe is enabled. >>>>>> >>>>>> -- an uprobe is inserted into a test application and enabled. >>>>>> >>>>>> So, when uprobe is enabled and test code execution reaches to probe >>>>>> instruction, it executes uprobe breakpoint instruction and el0_dbg >>>>>> exception is raised. >>>>>> >>>>>> When control reaches to start of uprobe_breakpoint_handler and it >>>>>> executes first instruction (which has been replaced with a kprobe >>>>>> breakpoint instruction), el1_dbg exception is raised. >>>>> >>>>> Hmm, debug exceptions should be masked at this point so I don't see why >>>>> you're taking the second debug exception. >>>>> >>>> >>>> So, you mean to say that when an exception which has been taken from >>>> lower exception level (EL0) is being executed, then we keep masked also >>>> the exception from current exception level (EL1)... >>> >>> Yeah, if you look at entry.S then you'll see that neither el0_dbg or el1_dbg >>> re-enable debug exceptions (masked automatically by the CPU after taking the >>> exception) until *after* the handling has completed. This is to prevent >>> recursive debug exceptions, which I don't see how we can reasonable handle. >> >> May be I am missing something, but my observation on silicon is >> different. Please have a look at git log of HEAD of following branch, >> which says that el1_dbg exception has been raised while el0_dbg was >> executing. Do not know what I am missing.. >> >> https://github.com/pratyushanand/linux/tree/ml_arm64_uprobe_devel_debug_kprobe_insertion_at_uprobe_breakpoint_handler > > That page just says "Failed to load latest commit information." for me. may be you can fetch https://github.com/pratyushanand/linux.git and can see git log of HEAD of ml_arm64_uprobe_devel_debug_kprobe_insertion_at_uprobe_breakpoint_handler. Or, you can apply attached patches on top of v3.18 kernel. > > Regardless, I think you need to debug further and found out if PSTATE.D is > getting cleared and, if so, who is responsible for that. Somebody could be > enabling IRQs, for example, which will then unmask debug exceptions in > el1_irq. > This is what I see for pstate, When el0_dbg exception is raised (ie an exception raised with ESR = ESR_EL1_EC_BRK64 after executing instruction BRK64_OPCODE_UPROBES = 0xD4200100 in EL0, user mode), spsr_el1 value is 0x80000000. Which means, all exceptions are unmasked. Is it expected? ~Pratyush -------------- next part -------------- A non-text attachment was scrubbed... Name: uprobe_kprobe_patches_over_v3.18.tar.bz2 Type: application/x-bzip Size: 26669 bytes Desc: not available URL: