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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
This commit is contained in:
Borislav Petkov 2018-04-12 13:11:36 +02:00
parent 71a93c2693
commit 093ae8f9a8
1 changed files with 14 additions and 2 deletions
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16
arch/x86/include/asm/msr.h
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@ -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)