Re: [PATCH v4 2/2] x86, mwaitt: introduce mwaix delay with a configurable timer

[Borislav Petkov]

On Thu, Jun 11, 2015 at 10:41:02PM +0800, Huang Rui wrote:
> MWAITX can enable a timer and a corresponding timer value specified in SW
> P0 clocks. The SW P0 frequency is the same with TSC. The timer provides an
> upper bound on how long the instruction waits before exiting.
> 
> The implementation of delay function in kernel can lerverage the timer of
> MWAITX. This patch provides a new method (delay_mwaitx) to measure delay
> time.
> 
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Suggested-by: Borislav Petkov <bp@xxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Huang Rui <ray.huang@xxxxxxx>
> ---
>  arch/x86/include/asm/mwait.h | 11 +++++++++++
>  arch/x86/lib/delay.c         | 41 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 51 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
> index 1fbc89d..9c8cccd 100644
> --- a/arch/x86/include/asm/mwait.h
> +++ b/arch/x86/include/asm/mwait.h
> @@ -14,6 +14,8 @@
>  #define CPUID5_ECX_INTERRUPT_BREAK	0x2
>  
>  #define MWAIT_ECX_INTERRUPT_BREAK	0x1
> +#define MWAITX_ECX_TIMER_ENABLE		BIT(1)
> +#define MWAITX_MAX_LOOPS		((u32)-1)
>  
>  static inline void __monitor(const void *eax, unsigned long ecx,
>  			     unsigned long edx)
> @@ -104,4 +106,13 @@ static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
>  	current_clr_polling();
>  }
>  
> +static inline void mwaitx(unsigned long eax, unsigned long loops,
> +			  bool enable)
> +{
> +	if (enable)
> +		__mwaitx(eax, loops, MWAITX_ECX_TIMER_ENABLE);
> +	else
> +		__mwaitx(eax, 0, 0);
> +}
> +
>  #endif /* _ASM_X86_MWAIT_H */
> diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
> index 39d6a3d..9daf94a 100644
> --- a/arch/x86/lib/delay.c
> +++ b/arch/x86/lib/delay.c
> @@ -20,6 +20,7 @@
>  #include <asm/processor.h>
>  #include <asm/delay.h>
>  #include <asm/timer.h>
> +#include <asm/mwait.h>
>  
>  #ifdef CONFIG_SMP
>  # include <asm/smp.h>
> @@ -87,6 +88,41 @@ static void delay_tsc(unsigned long __loops)
>  }
>  
>  /*
> + * On AMD platforms mwaitx has a configurable 32-bit timer, that counts
> + * with TSC frequency. And the input value is the loop of the counter, it
> + * will exit with the timer expired.
> + */
> +static void delay_mwaitx(unsigned long __loops)
> +{
> +	u32 end, start, delay, addr, loops = __loops;
> +
> +	rdtsc_barrier();
> +	rdtscl(start);
> +
> +	for (;;) {
> +		delay = min(MWAITX_MAX_LOOPS, loops);
> +
> +		__monitorx(&addr, 0, 0);
> +		/*
> +		 * AMD, like Intel, supports the EAX hint and EAX=0xf
> +		 * means, do not enter any deep C-state and we use it
> +		 * here in delay() to minimize wakeup latency.
> +		 */
> +		mwaitx(0xf, delay, true);
> +
> +		rdtsc_barrier();
> +		rdtscl(end);
> +
> +		if (loops <= end - start)
> +			break;
> +
> +		loops -= end - start;
> +
> +		start = end;
> +	}
> +}
> +
> +/*
>   * Since we calibrate only once at boot, this
>   * function should be set once at boot and not changed
>   */
> @@ -108,7 +144,10 @@ int read_current_timer(unsigned long *timer_val)
>  
>  void __delay(unsigned long loops)
>  {
> -	delay_fn(loops);
> +	if (!static_cpu_has_safe(X86_FEATURE_MWAITT))

Actually, as Thomas suggested, you don't need that static_cpu_has_safe()
thing either. You can change use_tsc_delay() to do this:

void use_tsc_delay(void)
{
	if (delay_fn == delay_loop)
		delay_fn = delay_tsc;
}

and then add:

void use_mwaitx_delay(void)
{
	delay_fn = delay_mwaitx;
}

and in cpu/amd.c:init_amd() do:

	if (static_cpu_has_safe(X86_FEATURE_MWAITT))
		use_mwaitx_delay();

Something like that, at least. tglx might have a better idea though.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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