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

From: "Woodhouse, David" <dwmw@amazon.co.uk>
To: Tom Lendacky <thomas.lendacky@amd.com>, <x86@kernel.org>,
	<linux-kernel@vger.kernel.org>,
	"Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
	"asit.k.mallick@intel.com" <asit.k.mallick@intel.com>
Cc: Rik van Riel <riel@redhat.com>, Andi Kleen <ak@linux.intel.com>,
	"Josh Poimboeuf" <jpoimboe@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jiri Kosina <jikos@kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Kees Cook <keescook@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	"Greg Kroah-Hartman" <gregkh@linux-foundation.org>,
	Paul Turner <pjt@google.com>
Subject: Re: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling RSB macros
Date: Sat, 13 Jan 2018 10:46:09 +0000	[thread overview]
Message-ID: <1515840369.22302.523.camel@amazon.co.uk> (raw)
In-Reply-To: <20180113010728.27928.8537.stgit@tlendack-t1.amdoffice.net>

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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@amd.com>

Conditionally-Acked-by: David Woodhouse <dwmw@amazon.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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