linux-kernel - Re: [RFC PATCH v2 1/1] arm64: Implement stack trace termination record

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Date:   Sat, 3 Apr 2021 23:40:51 -0500
From:   "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     mark.rutland@....com, broonie@...nel.org, jthierry@...hat.com,
        catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 1/1] arm64: Implement stack trace termination
 record

On 4/3/21 10:46 PM, Madhavan T. Venkataraman wrote:
>> I'm somewhat arm-ignorant, so take the following comments with a grain
>> of salt.
>>
>>
>> I don't think changing these to 'bl' is necessary, unless you wanted
>> __primary_switched() and __secondary_switched() to show up in the
>> stacktrace for some reason?  If so, that seems like a separate patch.
>>
> The problem is with __secondary_switched. If you trace the code back to where
> a secondary CPU is started, I don't see any calls anywhere. There are only
> branches if I am not mistaken. So, the return address register never gets
> set up with a proper address. The stack trace shows some hexadecimal value
> instead of a symbol name.
> 

Actually, I take that back. There are calls in that code path. But I did only
see some hexadecimal value instead of a proper address in the stack trace.
Sorry about that confusion.

My reason to convert the branches to calls is this - the value of the return
address register at that point is the return PC of the previous branch and link
instruction wherever that happens to be. I think that is a little arbitrary.

Instead, if I call start_kernel() and secondary_start_kernel(), the return address
gets set up to the next instruction which, IMHO, is better.

But I am open to other suggestions.

Madhavan