[RFC PATCH 4/4] x86/TSC: Use RDTSCP

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

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 and some are 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 <tglx@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Link: https://lkml.kernel.org/r/20181119184556.11479-1-bp@xxxxxxxxx
---
 arch/x86/include/asm/msr.h | 16 ++++++++++++++--
 1 file 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)
-- 
2.19.1