linux-kernel - Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

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Date:   Thu, 4 Jan 2018 10:17:46 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     ak@...ux.intel.com, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v3 01/13] x86/retpoline: Add initial retpoline support

On Thu, Jan 04, 2018 at 02:36:58PM +0000, David Woodhouse wrote:
> Enable the use of -mindirect-branch=thunk-extern in newer GCC, and provide
> the corresponding thunks. Provide assembler macros for invoking the thunks
> in the same way that GCC does, from native and inline assembler.
> 
> This adds an X86_BUG_NO_RETPOLINE "feature" for runtime patching out
> of the thunks. This is a placeholder for now; the patches which support
> the new Intel/AMD microcode features will flesh out the precise conditions
> under which we disable the retpoline and do other things instead.
> 
> [Andi Kleen: Rename the macros and add CONFIG_RETPOLINE option]
> 
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
...
> +.macro THUNK sp reg
> +	.section .text.__x86.indirect_thunk.\reg
> +
> +ENTRY(__x86.indirect_thunk.\reg)
> +	CFI_STARTPROC
> +	ALTERNATIVE "call 2f", __stringify(jmp *%\reg), X86_BUG_NO_RETPOLINE
> +1:
> +	lfence
> +	jmp	1b
> +2:
> +	mov	%\reg, (%\sp)
> +	ret
> +	CFI_ENDPROC
> +ENDPROC(__x86.indirect_thunk.\reg)

Clearly Paul's approach to retpoline without lfence is faster.
I'm guessing it wasn't shared with amazon/intel until now and
this set of patches going to adopt it, right?

Paul, could you share a link to a set of alternative gcc patches
that do retpoline similar to llvm diff ?