linux-kernel - Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling RSB macros

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Date:   Sat, 13 Jan 2018 10:46:09 +0000
From:   "Woodhouse, David" <dwmw@...zon.co.uk>
To:     Tom Lendacky <thomas.lendacky@....com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>,
        "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>,
        "asit.k.mallick@...el.com" <asit.k.mallick@...el.com>
CC:     Rik van Riel <riel@...hat.com>, Andi Kleen <ak@...ux.intel.com>,
        "Josh Poimboeuf" <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "Greg Kroah-Hartman" <gregkh@...ux-foundation.org>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB
 filling RSB macros

On Fri, 2018-01-12 at 19:07 -0600, Tom Lendacky wrote:
> The pause instruction is currently used in the retpoline and RSB filling
> macros as a speculation trap.  The use of pause was originally suggested
> because it showed a very, very small difference in the amount of
> cycles/time used to execute the retpoline as compared to lfence.  On AMD,
> the pause instruction is not a serializing instruction, so the pause/jmp
> loop will use excess power as it is speculated over waiting for return
> to mispredict to the correct target.
> 
> The RSB filling macro is applicable to AMD, and, if software is unable to
> verify that lfence is serializing on AMD (possible when running under a
> hypervisor), the generic retpoline support will be used and, so, is also
> applicable to AMD.  Change the use of pause to lfence.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>

Conditionally-Acked-by: David Woodhouse <dwmw@...zon.co.uk>

The condition being, as noted, that I'd really like to see it acked by
Arjan/Asit and Paul.



> ---
>  arch/x86/include/asm/nospec-branch.h |   10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 402a11c..2c4a09a 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -11,7 +11,7 @@
>   * Fill the CPU return stack buffer.
>   *
>   * Each entry in the RSB, if used for a speculative 'ret', contains an
> - * infinite 'pause; jmp' loop to capture speculative execution.
> + * infinite 'lfence; jmp' loop to capture speculative execution.
>   *
>   * This is required in various cases for retpoline and IBRS-based
>   * mitigations for the Spectre variant 2 vulnerability. Sometimes to
> @@ -37,12 +37,12 @@
>  771:						\
>  	call	772f;				\
>  773:	/* speculation trap */			\
> -	pause;					\
> +	lfence;					\
>  	jmp	773b;				\
>  772:						\
>  	call	774f;				\
>  775:	/* speculation trap */			\
> -	pause;					\
> +	lfence;					\
>  	jmp	775b;				\
>  774:						\
>  	dec	reg;				\
> @@ -72,7 +72,7 @@
>  .macro RETPOLINE_JMP reg:req
>  	call	.Ldo_rop_\@
>  .Lspec_trap_\@:
> -	pause
> +	lfence
>  	jmp	.Lspec_trap_\@
>  .Ldo_rop_\@:
>  	mov	\reg, (%_ASM_SP)
> @@ -164,7 +164,7 @@
>  	"       jmp    904f;\n"					\
>  	"       .align 16\n"					\
>  	"901:	call   903f;\n"					\
> -	"902:	pause;\n"					\
> +	"902:	lfence;\n"					\
>  	"       jmp    902b;\n"					\
>  	"       .align 16\n"					\
>  	"903:	addl   $4, %%esp;\n"				\
> 
> 
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