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SubjectRe: [PATCH v1] x86/retpoline: Use lfence in the retpoline/RSB filling RSB macros
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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" \
>
> [unhandled content-type:application/x-pkcs7-signature]
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