[PATCH v1 2/2] x86/cpu/AMD: Clear RETPOLINE_AMD if LFENCE is not serializing

[Tom Lendacky <thomas.lendacky@amd.com>]

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();
 		}
 	}