Re: [PATCH] x86/TSC: Use RDTSCP

["Lendacky, Thomas" <Thomas.Lendacky@amd.com>]

On 11/19/2018 12:45 PM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@suse.de>
> 
> 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.

Thanks for this. I had been wanting to do something similar since, on AMD,
a guest may not see the LFENCE-serializing bit if the hypervisor doesn't
expose it, and have to fall back to MFENCE.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

> 
> 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: Andy Lutomirski <luto@kernel.org>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Thomas Lendacky <Thomas.Lendacky@amd.com>
> Cc: x86@kernel.org
> ---
>  arch/x86/include/asm/msr.h | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 91e4cf189914..f00f2b61d326 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 +229,18 @@ 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_2("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)
>