3.16.56-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Tom Lendacky <thomas.lenda...@amd.com>

commit 9c6a73c75864ad9fa49e5fa6513e4c4071c0e29f upstream.

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.lenda...@amd.com>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
Reviewed-by: Reviewed-by: Borislav Petkov <b...@suse.de>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Tim Chen <tim.c.c...@linux.intel.com>
Cc: Dave Hansen <dave.han...@intel.com>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Dan Williams <dan.j.willi...@intel.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Greg Kroah-Hartman <gre...@linux-foundation.org>
Cc: David Woodhouse <d...@amazon.co.uk>
Cc: Paul Turner <p...@google.com>
Link: 
https://lkml.kernel.org/r/20180108220932.12580.52458.st...@tlendack-t1.amdoffice.net
[bwh: Backported to 3.16: adjust filename, context]
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
 arch/x86/include/uapi/asm/msr-index.h |  1 +
 arch/x86/kernel/cpu/amd.c             | 18 ++++++++++++++++--
 2 files changed, 17 insertions(+), 2 deletions(-)

--- a/arch/x86/include/uapi/asm/msr-index.h
+++ b/arch/x86/include/uapi/asm/msr-index.h
@@ -225,6 +225,7 @@
 #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
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -673,6 +673,9 @@ static void init_amd(struct cpuinfo_x86
                set_cpu_cap(c, X86_FEATURE_K8);
 
        if (cpu_has_xmm2) {
+               unsigned long long val;
+               int ret;
+
                /*
                 * A serializing LFENCE has less overhead than MFENCE, so
                 * use it for execution serialization.  On families which
@@ -683,8 +686,19 @@ static void init_amd(struct cpuinfo_x86
                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);
+               }
        }
 
 #ifdef CONFIG_X86_64

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