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Date:   Thu, 24 Feb 2022 10:42:30 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     x86@kernel.org, joao@overdrivepizza.com, hjl.tools@gmail.com,
        jpoimboe@redhat.com, andrew.cooper3@citrix.com,
        linux-kernel@vger.kernel.org, ndesaulniers@google.com,
        keescook@chromium.org, samitolvanen@google.com,
        mark.rutland@arm.com, alyssa.milburn@intel.com, mbenes@suse.cz,
        mhiramat@kernel.org, alexei.starovoitov@gmail.com
Subject: Re: [PATCH v2 14/39] x86/ibt,ftrace: Make function-graph play nice
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On Thu, 24 Feb 2022 16:36:57 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Feb 24, 2022 at 03:51:52PM +0100, Peter Zijlstra wrote:
> > @@ -316,10 +317,12 @@ SYM_FUNC_START(return_to_handler)
> >  
> >  	call ftrace_return_to_handler
> >  
> > -	movq %rax, %rdi
> > +	movq %rax, 16(%rsp)
> >  	movq 8(%rsp), %rdx
> >  	movq (%rsp), %rax
> > -	addq $24, %rsp
> > -	JMP_NOSPEC rdi
> > +
> > +	addq $16, %rsp
> > +	UNWIND_HINT_FUNC
> > +	RET
> >  SYM_FUNC_END(return_to_handler)
> >  #endif  
> 
> While talking about this with Mark, an alternative solution is something
> like this, that would keep the RSB balanced and only mess up the current
> return.
> 
> No idea it if makes an appreciatable difference on current hardware,
> therefore I went with the simpler option above.
> 
> @@ -307,7 +315,7 @@ EXPORT_SYMBOL(__fentry__)
>  
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  SYM_FUNC_START(return_to_handler)
> -	subq  $24, %rsp
> +	subq  $16, %rsp
>  
>  	/* Save the return values */
>  	movq %rax, (%rsp)
> @@ -319,7 +327,13 @@ SYM_FUNC_START(return_to_handler)
>  	movq %rax, %rdi
>  	movq 8(%rsp), %rdx
>  	movq (%rsp), %rax
> -	addq $24, %rsp
> -	JMP_NOSPEC rdi
> +
> +	addq $16, %rsp
> +	ANNOTATE_INTRA_FUNCTION_CALL
> +	call .Ldo_rop
> +.Ldo_rop:

What's the overhead of an added call (for every function call that is being
traced)?

-- Steve

> +	mov %rdi, (%rsp)
> +	UNWIND_HINT_FUNC
> +	RET
>  SYM_FUNC_END(return_to_handler)
>  #endif
>