linux-kernel - Re: [RFC PATCH 4/4] x86/TSC: Use RDTSCP

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Date:   Wed, 12 Dec 2018 02:37:53 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     luto@...nel.org
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, bp@...e.de,
        thomas.lendacky@....com, hpa@...or.com, john.stultz@...aro.org,
        x86@...nel.org
Subject: Re: [RFC PATCH 4/4] x86/TSC: Use RDTSCP

> And I had a bit of a WTF moment, as in "WTF does
> RDTSC have to do with a speculation protection barrier".
> Does it actually make sense?

It doesn't. There was too much s/lfence/barrier_nospec/ apparently.

> +	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");

I have a question: does alternatives ordering matter? CPU can have
both features.

And other code in this file uses "c" for clobber.