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

[Dan Williams]

On Fri, Jan 12, 2018 at 5:07 PM, Tom Lendacky <thomas.lendacky@xxxxxxx> 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.

Should we use ASM_IFENCE for this?