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

["Woodhouse, David" <dwmw@amazon.co.uk>]

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On Sun, 2018-01-14 at 16:41 +0100, Borislav Petkov wrote:
> 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@amd.com>
> > ---
> >  arch/x86/include/asm/nospec-branch.h |    6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
>
> Reviewed-by: Borislav Petkov <bp@suse.de>

Acked-by: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: David Woodhouse <dwmw@amazon.co.uk>

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