[PATCH v2 2/2] x86/cpu/AMD: Use LFENCE_RDTSC in preference to MFENCE_RDTSC

[Tom Lendacky]

With LFENCE now a serializing instruction, use LFENCE_RDTSC in preference
to MFENCE_RDTSC.  However, since the kernel could be running under a
hypervisor that does not support writing that MSR, read the MSR back and
verify that the bit has been set successfully.  If the MSR can be read
and the bit is set, then set the LFENCE_RDTSC feature, otherwise set the
MFENCE_RDTSC feature.

Signed-off-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
---
 arch/x86/include/asm/msr-index.h |    1 +
 arch/x86/kernel/cpu/amd.c        |   17 +++++++++++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 1e7d710..fa11fb1 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -354,6 +354,7 @@
 #define MSR_FAM10H_NODE_ID		0xc001100c
 #define MSR_F10H_DECFG			0xc0011029
 #define MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT	1
+#define MSR_F10H_DECFG_LFENCE_SERIALIZE		BIT_ULL(MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT)
 
 /* K8 MSRs */
 #define MSR_K8_TOP_MEM1			0xc001001a
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 5b438d8..053f6c7 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -829,6 +829,9 @@ static void init_amd(struct cpuinfo_x86 *c)
 		set_cpu_cap(c, X86_FEATURE_K8);
 
 	if (cpu_has(c, X86_FEATURE_XMM2)) {
+		unsigned long long val;
+		int ret;
+
 		/*
 		 * A serializing LFENCE has less overhead than MFENCE, so
 		 * use it for execution serialization.  On families which
@@ -839,8 +842,18 @@ static void init_amd(struct cpuinfo_x86 *c)
 		msr_set_bit(MSR_F10H_DECFG,
 			    MSR_F10H_DECFG_LFENCE_SERIALIZE_BIT);
 
-		/* MFENCE stops RDTSC speculation */
-		set_cpu_cap(c, X86_FEATURE_MFENCE_RDTSC);
+		/*
+		 * Verify that the MSR write was successful (could be running
+		 * under a hypervisor) and only then assume that LFENCE is
+		 * serializing.
+		 */
+		ret = rdmsrl_safe(MSR_F10H_DECFG, &val);
+		if (!ret && (val & MSR_F10H_DECFG_LFENCE_SERIALIZE))
+			/* A serializing LFENCE stops RDTSC speculation */
+			set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC);
+		else
+			/* MFENCE stops RDTSC speculation */
+			set_cpu_cap(c, X86_FEATURE_MFENCE_RDTSC);
 	}
 
 	/*