linux-kernel - Re: [PATCH v3 3/3] x86/delay: Introduce TPAUSE delay

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Message-ID: <20200414180545.GE2483@worktop.programming.kicks-ass.net>
Date:   Tue, 14 Apr 2020 20:05:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     "Park, Kyung Min" <kyung.min.park@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>
Subject: Re: [PATCH v3 3/3] x86/delay: Introduce TPAUSE delay

On Tue, Apr 14, 2020 at 06:00:21PM +0000, Luck, Tony wrote:
> >> +static inline void __tpause(u32 ecx, u32 edx, u32 eax)
> >> +{
> >> +	/* "tpause %ecx, %edx, %eax;" */
> >> +	asm volatile(".byte 0x66, 0x0f, 0xae, 0xf1\t\n"
> >> +		     :
> >> +		     : "c"(ecx), "d"(edx), "a"(eax));
> >> +}
> >
> > Can we please get a comment stating from what binutils version this
> > opcode has a mnemonic? That way, when we raise the minimum binutils
> > version we can easily grep and find such things.
> 
> Or maybe use arch/x86/Kconfig.assembler to set up a CONFIG_AS_TPAUSE?
> 
> Then the code can read something like (syntax may need fixing)
> 
> #ifdef CONFIG_AS_TPAUSE
> 		asm volatile("tpause %ecx\n", : : "c"(ecx), "d"(edx), "a"(eax));
> #else
> 		asm volatile(".byte hex gibberish ...
> #endif

Then we still need a comment to know when we can kill that...