path: root/arch/x86/include/asm
diff options
authorBorislav Petkov <>2018-04-12 13:11:36 +0200
committerBorislav Petkov <>2019-01-16 12:43:08 +0100
commit093ae8f9a86a974c920b613860f1f7fd5bbd70ab (patch)
tree645fe92d87a76409fa3ae942d816e80341c26c3c /arch/x86/include/asm
parent71a93c26930471e976dd184ef91931b2a5393afc (diff)
Currently, the kernel uses [LM]FENCE; RDTSC in the timekeeping code, to guarantee monotonicity of time where the *FENCE is selected based on vendor. Replace that sequence with RDTSCP which is faster or on-par and gives the same guarantees. A microbenchmark on Intel shows that the change is on-par. On AMD, the change is either on-par with the current LFENCE-prefixed RDTSC or slightly better with RDTSCP. The comparison is done with the LFENCE-prefixed RDTSC (and not with the MFENCE-prefixed one, as one would normally expect) because all modern AMD families make LFENCE serializing and thus avoid the heavy MFENCE by effectively enabling X86_FEATURE_LFENCE_RDTSC. Co-developed-by: Thomas Gleixner <> Signed-off-by: Thomas Gleixner <> Signed-off-by: Borislav Petkov <> Cc: Tom Lendacky <> Cc: Andy Lutomirski <> Cc: "H. Peter Anvin" <> Cc: John Stultz <> Cc: Link:
Diffstat (limited to 'arch/x86/include/asm')
1 files changed, 14 insertions, 2 deletions
diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 91e4cf189914..5cc3930cb465 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -217,6 +217,8 @@ static __always_inline unsigned long long rdtsc(void)
static __always_inline unsigned long long rdtsc_ordered(void)
+ DECLARE_ARGS(val, low, high);
* The RDTSC instruction is not ordered relative to memory
* access. The Intel SDM and the AMD APM are both vague on this
@@ -227,9 +229,19 @@ static __always_inline unsigned long long rdtsc_ordered(void)
* ordering guarantees as reading from a global memory location
* that some other imaginary CPU is updating continuously with a
* time stamp.
+ *
+ * Thus, use the preferred barrier on the respective CPU, aiming for
+ * RDTSCP as the default.
- barrier_nospec();
- return rdtsc();
+ asm volatile(ALTERNATIVE_3("rdtsc",
+ "mfence; rdtsc", X86_FEATURE_MFENCE_RDTSC,
+ "lfence; rdtsc", X86_FEATURE_LFENCE_RDTSC,
+ "rdtscp", X86_FEATURE_RDTSCP)
+ : EAX_EDX_RET(val, low, high)
+ /* RDTSCP clobbers ECX with MSR_TSC_AUX. */
+ :: "ecx");
+ return EAX_EDX_VAL(val, low, high);
static inline unsigned long long native_read_pmc(int counter)