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Date:   Fri, 4 Mar 2022 22:44:54 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     x86@kernel.org, joao@overdrivepizza.com, hjl.tools@gmail.com,
        andrew.cooper3@citrix.com, linux-kernel@vger.kernel.org,
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        samitolvanen@google.com, mark.rutland@arm.com,
        alyssa.milburn@intel.com, mbenes@suse.cz, rostedt@goodmis.org,
        mhiramat@kernel.org, alexei.starovoitov@gmail.com
Subject: Re: [PATCH v3 18/39] x86/ibt,ftrace: Make function-graph play nice
Message-ID: <20220304214454.GT11184@worktop.programming.kicks-ass.net>
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On Fri, Mar 04, 2022 at 01:03:34PM -0800, Josh Poimboeuf wrote:
> On Fri, Mar 04, 2022 at 08:48:53PM +0100, Peter Zijlstra wrote:
> > On Fri, Mar 04, 2022 at 09:51:54AM -0800, Josh Poimboeuf wrote:
> > > On Thu, Mar 03, 2022 at 12:23:39PM +0100, Peter Zijlstra wrote:
> > > > +
> > > > +	addq $16, %rsp
> > > > +	ANNOTATE_INTRA_FUNCTION_CALL
> > > > +	call .Ldo_rop
> > > > +	int3
> > > > +.Ldo_rop:
> > > > +	mov %rdi, (%rsp)
> > > > +	UNWIND_HINT_FUNC
> > > > +	RET
> > > 
> > > Why the int3?
> > 
> > Speculation trap :-) Either I'm too paranoid or not paranoid enough; but
> > without it it's just too close to a retpoline and it doesn't feel right.
> 
> Um, it *is* a retpoline :-)
> 
> Can you just use the RETPOLINE macro?  Along with a comment stating why
> it can't just do a JMP_NOSPEC?

There is no RETPOLINE macro; or rather it is completely contained in
lib/retpoline.S and I'd sorta like to keep it that way.

That said, I can stick a comment on.