Re: [PATCH v2] x86/retpoline: Add LFENCE to the retpoline/RSB filling RSB macros

[Borislav Petkov]

On Sat, Jan 13, 2018 at 05:27:30PM -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.  Keep the current usage of PAUSE for Intel, but add an
> LFENCE instruction to the speculation trap for AMD.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> ---
>  arch/x86/include/asm/nospec-branch.h |    6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)

Reviewed-by: Borislav Petkov <bp@xxxxxxx>

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.