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Date: Sun, 14 Jan 2018 16:41:08 +0100
From: Borislav Petkov <bp@alien8.de>
To: Tom Lendacky <thomas.lendacky@amd.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
        Rik van Riel <riel@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jiri Kosina <jikos@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Paul Turner <pjt@google.com>
Subject: Re: [PATCH v2] x86/retpoline: Add LFENCE to the retpoline/RSB
 filling RSB macros
Message-ID: <20180114154108.3og4oqascfawy4oe@pd.tnic>
References: <20180113232730.31060.36287.stgit@tlendack-t1.amdoffice.net>
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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>

-- 
Regards/Gruss,
    Boris.

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