From mboxrd@z Thu Jan  1 00:00:00 1970
From: panand@redhat.com (Pratyush Anand)
Date: Fri, 16 Jan 2015 17:30:09 +0530
Subject: Query: ARM64: Behavior of el1_dbg exception while executing
 el0_dbg
In-Reply-To: <54B7EF3C.8060909@redhat.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>
 <54B4BF4C.2030009@redhat.com>
 <20150113155221.GG16524@e104818-lin.cambridge.arm.com>
 <54B55B95.1070402@redhat.com> <54B7EF3C.8060909@redhat.com>
Message-ID: <54B8FD49.2060503@redhat.com>
To: linux-arm-kernel@lists.infradead.org
List-Id: linux-arm-kernel.lists.infradead.org

Hi Will,


On Thursday 15 January 2015 10:17 PM, Pratyush Anand wrote:
> Hi Will / Catalin,
>
> 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.

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].

~Pratyush