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

Alexey Dobriyan
2018-12-11T23:37:59Z

> 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.