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

[Woodhouse, David]

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.ÂÂThe 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.ÂÂOn 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.ÂÂChange the use of pause to lfence.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

Conditionally-Acked-by: David Woodhouse <dwmw@xxxxxxxxxxxx>

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



> ---
> Âarch/x86/include/asm/nospec-branch.h |ÂÂÂ10 +++++-----
> Â1 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 @@
> Â * Fill the CPU return stack buffer.
> Â *
> Â * 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.
> Â *
> Â * This is required in various cases for retpoline and IBRS-based
> Â * mitigations for the Spectre variant 2 vulnerability. Sometimes to
> @@ -37,12 +37,12 @@
> Â771:						\
> Â	call	772f;				\
> Â773:	/* speculation trap */			\
> -	pause;					\
> +	lfence;					\
> Â	jmp	773b;				\
> Â772:						\
> Â	call	774f;				\
> Â775:	/* speculation trap */			\
> -	pause;					\
> +	lfence;					\
> Â	jmp	775b;				\
> Â774:						\
> Â	dec	reg;				\
> @@ -72,7 +72,7 @@
> Â.macro RETPOLINE_JMP reg:req
> Â	call	.Ldo_rop_\@
> Â.Lspec_trap_\@:
> -	pause
> +	lfence
> Â	jmp	.Lspec_trap_\@
> Â.Ldo_rop_\@:
> Â	mov	\reg, (%_ASM_SP)
> @@ -164,7 +164,7 @@
> Â	"ÂÂÂÂÂÂÂjmpÂÂÂÂ904f;\n"					\
> Â	"ÂÂÂÂÂÂÂ.align 16\n"					\
> Â	"901:	callÂÂÂ903f;\n"					\
> -	"902:	pause;\n"					\
> +	"902:	lfence;\n"					\
> Â	"ÂÂÂÂÂÂÂjmpÂÂÂÂ902b;\n"					\
> Â	"ÂÂÂÂÂÂÂ.align 16\n"					\
> Â	"903:	addlÂÂÂ$4, %%esp;\n"				\
> 
>

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