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Subject: Re: [PATCH] powerpc/64: Avoid link stack corruption in kexec_wait()
To:     Daniel Axtens <dja@axtens.net>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
References: <3ffe7775f3fcda8e5fca6a7bc7db0b8251153c67.1629705147.git.christophe.leroy@csgroup.eu>
 <87ilzm6str.fsf@dja-thinkpad.axtens.net>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <9f43118b-ef81-9a80-0e0b-5e74433e0b8c@csgroup.eu>
Date:   Tue, 31 Aug 2021 10:54:38 +0200
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Le 31/08/2021 à 08:17, Daniel Axtens a écrit :
> Hi Christophe,
> 
>> Use bcl 20,31,+4 instead of bl in order to preserve link stack.
>>
>> See commit c974809a26a1 ("powerpc/vdso: Avoid link stack corruption
>> in __get_datapage()") for details.
> 
>  From my understanding of that commit message, the change helps to keep
> the link stack correctly balanced which is helpful for performance,
> rather than for correctness. If I understand correctly, kexec_wait is
> not in a hot path - rather it is where CPUs spin while waiting for
> kexec. Is there any benefit in using the more complicated opcode in this
> situation?

AFAICS the main benefit is to keep things consistent over the kernel and not have to wonder "is it a 
hot path or not ? If it is I use bcl 20,31, if it is not I use bl". The best way to keep things in 
order is to always use the right instruction.

> 
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
>>   arch/powerpc/kernel/misc_64.S | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/kernel/misc_64.S b/arch/powerpc/kernel/misc_64.S
>> index 4b761a18a74d..613509907166 100644
>> --- a/arch/powerpc/kernel/misc_64.S
>> +++ b/arch/powerpc/kernel/misc_64.S
>> @@ -255,7 +255,7 @@ _GLOBAL(scom970_write)
>>    * Physical (hardware) cpu id should be in r3.
>>    */
>>   _GLOBAL(kexec_wait)
>> -	bl	1f
>> +	bcl	20,31,1f
>>   1:	mflr	r5
> 
> Would it be better to create a macro of some sort to wrap this unusual
> special form so that the meaning is more clear?

Not sure, I think people working with assembly will easily recognise that form whereas an obscure 
macro is always puzzling.

I like macros when they allow you to not repeat again and again the same sequence of several 
instructions, but here it is a single quite simple instruction which is not worth a macro in my mind.

Christophe