From: Tom Lendacky <thomas.lendacky@amd.com>
Subject: [PATCH v1 2/2] x86/cpu/AMD: Clear RETPOLINE_AMD if LFENCE is not
 serializing
To: x86@kernel.org, linux-kernel@vger.kernel.org
Cc: Rik van Riel <riel@redhat.com>, Andi Kleen <ak@linux.intel.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>,
        Borislav Petkov <bp@alien8.de>,
        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>
Date: Tue, 09 Jan 2018 16:37:45 -0600
Message-ID: <20180109223745.29322.30862.stgit@tlendack-t1.amdoffice.net>
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References: <20180109223723.29322.59126.stgit@tlendack-t1.amdoffice.net>
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During early boot, spectre_v2_check_boottime_disable() sets the
RETPOLINE_AMD feature for AMD hardware (before init_amd() is called).
Since the RETPOLINE_AMD feature uses LFENCE to perform speculation
control, it can only be used if LFENCE is serializing.  Clear the
RETPOLINE_AMD feature if the MSR write to make LFENCE serializing
is not successful.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
---
 arch/x86/kernel/cpu/amd.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index ea831c8..8a0076b 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -13,6 +13,7 @@
 #include <asm/smp.h>
 #include <asm/pci-direct.h>
 #include <asm/delay.h>
+#include <asm/nospec-branch.h>
 
 #ifdef CONFIG_X86_64
 # include <asm/mmconfig.h>
@@ -854,6 +855,9 @@ static void init_amd(struct cpuinfo_x86 *c)
 		} else {
 			/* MFENCE stops RDTSC speculation */
 			set_cpu_cap(c, X86_FEATURE_MFENCE_RDTSC);
+
+			/* Don't use the LFENCE-based retpoline support */
+			retpoline_amd_disable();
 		}
 	}