linux-kernel - Re: [PATCH v2 2/2] x86/delay: Introduce TPAUSE delay

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Message-ID: <b771dfc7409a99b35575c14cd4dd55d24f81ca98.camel@perches.com>
Date:   Fri, 20 Mar 2020 03:07:10 -0700
From:   Joe Perches <joe@...ches.com>
To:     Kyung Min Park <kyung.min.park@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        gregkh@...uxfoundation.org, ak@...ux.intel.com,
        tony.luck@...el.com, ashok.raj@...el.com, ravi.v.shankar@...el.com,
        fenghua.yu@...el.com
Subject: Re: [PATCH v2 2/2] x86/delay: Introduce TPAUSE delay

On Thu, 2020-03-19 at 21:13 -0700, Kyung Min Park wrote:
> TPAUSE instructs the processor to enter an implementation-dependent
> optimized state. The instruction execution wakes up when the time-stamp
> counter reaches or exceeds the implicit EDX:EAX 64-bit input value.
> The instruction execution also wakes up due to the expiration of
> the operating system time-limit or by an external interrupt
> or exceptions such as a debug exception or a machine check exception.
[]
> diff --git a/arch/x86/lib/delay.c b/arch/x86/lib/delay.c
[]
> @@ -97,6 +97,27 @@ static void delay_tsc(u64 cycles)
>  }
>  
>  /*
> + * On Intel the TPAUSE instruction waits until any of:
> + * 1) the TSC counter exceeds the value provided in EAX:EDX
> + * 2) global timeout in IA32_UMWAIT_CONTROL is exceeded
> + * 3) an external interrupt occurs
> + */
> +static void delay_halt_tpause(u64 start, u64 cycles)
> +{
> +	u64 until = start + cycles;
> +	unsigned int eax, edx;
> +
> +	eax = (unsigned int)(until & 0xffffffff);
> +	edx = (unsigned int)(until >> 32);

trivia:

perhaps lower_32_bits and upper_32_bits