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Subject: Re: [PATCH 4/7] objtool: Add support for return trampoline call
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Alexandre Chartre <alexandre.chartre@oracle.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, jpoimboe@redhat.com,
        tglx@linutronix.de
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From:   Julien Thierry <jthierry@redhat.com>
Message-ID: <bc3a31dc-9d59-5756-aad3-187533f05654@redhat.com>
Date:   Fri, 3 Apr 2020 09:11:55 +0100
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On 4/2/20 4:40 PM, Peter Zijlstra wrote:
> On Thu, Apr 02, 2020 at 04:31:05PM +0100, Julien Thierry wrote:
>> My understanding is that once you find an intra-function call, either you
>> hit a return, ending the branch, so the return should undo the modification
>> the intra-function call did (whether is it a retpoline return or not).
>> Otherwise, the intra-function call branch will need to reach an end in some
>> way (e.g. hitting a CONTEXT_SWITCH instruction, calling a
>> dead_end_function).
>>
>> Am I missing something?
> 
> The thing is basically doing:
> 
> 	mov  $n, cx
> 1:	call 2f
> 2:	dec  cx
> 	jnz  1b
> 	add  8*n, sp
> 
> So it does N calls to self, then subtracts N words from the stack.
> 
> The reason being that the CPU has a return-stack-buffer for predicting
> returns, and call/ret being naturally paired, that works. The above
> is a software flush of the RSB.
> 

Ah, lovely... Maybe that's where SAVE/RESTORE unwind hints could be nice 
;) .

Otherwise, I don't really have a good suggestion for this...

-- 
Julien Thierry