Subject: Query: ARM64: Behavior of el1_dbg exception while executing el0_dbg

From: [email-masked] (Pratyush Anand)
Date: Fri, 16 Jan 2015 17:30:09 +0530

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