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Subject: Re: [PATCH 4/7] objtool: Add support for return trampoline call
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        Julien Thierry <jthierry@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, tglx@linutronix.de
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From:   Alexandre Chartre <alexandre.chartre@oracle.com>
Message-ID: <0e8e9c1f-e03b-207e-a527-cf8deddf82a0@oracle.com>
Date:   Mon, 6 Apr 2020 13:03:36 +0200
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On 4/6/20 11:31 AM, Peter Zijlstra wrote:
> On Mon, Apr 06, 2020 at 10:19:56AM +0200, Alexandre Chartre wrote:
>>
>> On 4/4/20 5:51 PM, Peter Zijlstra wrote:
>>> On Sat, Apr 04, 2020 at 09:22:32AM -0500, Josh Poimboeuf wrote:
>>>> On Sat, Apr 04, 2020 at 03:32:18PM +0200, Peter Zijlstra wrote:
>>>>> On Fri, Apr 03, 2020 at 05:46:20PM +0200, Peter Zijlstra wrote:
>>>>>> On Fri, Apr 03, 2020 at 10:17:57AM -0500, Josh Poimboeuf wrote:
>>>>>>> Peter, I think my previous idea for UNWIND_HINT_ADJUST stack_add=8 would
>>>>>>> work here?
>>>>>>
>>>>>> Yes, it would.
>>>>>
>>>>> Sorry, I have reconsidered. While it will shut up objtool, it will not
>>>>> 'work'. That is, the ORC data generated will not correctly unwind.
>>>>>
>>>>> I'll try and write a longer email tonight.
>>>>
>>>> Right, that's what I've been trying to say.  The ORC data will be
>>>> non-deterministic unless we unroll the loop.  Or did you mean something
>>>> else?
>>>
>>> The below should result in deterministic code.
>>>
>>> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
>>> index 07e95dcb40ad..109ee65f4a11 100644
>>> --- a/arch/x86/include/asm/nospec-branch.h
>>> +++ b/arch/x86/include/asm/nospec-branch.h
>>> @@ -59,8 +59,8 @@
>>>    	jmp	775b;				\
>>>    774:						\
>>>    	dec	reg;				\
>>> -	jnz	771b;				\
>>> -	add	$(BITS_PER_LONG/8) * nr, sp;
>>> +	add	$(BITS_PER_LONG/8) * $2, sp;	\
>>> +	jnz	771b;
>>>    #ifdef __ASSEMBLY__
>>
>> Nice. This works fine and allows to remove ANNOTATE_NOSPEC_ALTERNATIVE when
>> using __FILL_RETURN_BUFFER. However this is probably less performant because
>> we now have nr/2 add instructions instead of just 1.
> 
> Does it actually matter though? That is, can you measure the difference?
> 

I didn't do any measurement, I am just anticipating concerns others might have
as this code is used during context and vmexit. But I agree this might not be
significant.

alex.