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| author | Borislav Petkov <bp@suse.de> | 2018-04-12 14:11:36 +0300 |
|---|---|---|
| committer | Borislav Petkov <bp@suse.de> | 2019-01-16 14:43:08 +0300 |
| commit | 093ae8f9a86a974c920b613860f1f7fd5bbd70ab (patch) | |
| tree | 645fe92d87a76409fa3ae942d816e80341c26c3c | |
| parent | 71a93c26930471e976dd184ef91931b2a5393afc (diff) | |
| download | linux-093ae8f9a86a974c920b613860f1f7fd5bbd70ab.tar.xz | |
x86/TSC: Use RDTSCP
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 <tglx@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: x86@kernel.org
Link: https://lkml.kernel.org/r/20181119184556.11479-1-bp@alien8.de
| -rw-r--r-- | arch/x86/include/asm/msr.h | 16 |
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) |