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

Borislav Petkov
2018-12-11T23:39:12Z

On Tue, Dec 11, 2018 at 11:12:41PM +0000, Lendacky, Thomas wrote:
> It does seem overloaded in that sense, but the feature means that LFENCE
> is serializing and so can be used in rdtsc_ordered. In the same sense,
> barrier_nospec is looking for whether LFENCE is serializing and preferring
> that over MFENCE since it is lighter weight.
>
> In light of how they're being used now, they could probably stand to be
> renamed in some way.

Actually, come to think of it, what really matters here is whether
LFENCE is serializing or not. Because if so, you wanna replace with LFENCE
as it is lighter. And in that case a single alternative() - not _2() -
should suffice.

BUT(!), that still is not good enough if you do some qemu CPU models
like pentium or so which don't even have MFENCE and cause stuff like
this:

https://lkml.kernel.org/r/20181123200307.GA6223@roeck-us.net

Which means, that you *do* have to alternate between

* no insn at all
* MFENCE
* LFENCE, if it is serializing

so barrier_nospec() does the right thing, AFAICS. And this is why we
need an ALTERNATIVE_3() to add RDTSCP into the mix too.

WRT renaming, I guess we can do something like:

* X86_FEATURE_MFENCE_RDTSC -> X86_FEATURE_MFENCE - to mean that CPU has
MFENCE support.

and

* X86_FEATURE_LFENCE_RDTSC -> X86_FEATURE_LFENCE_SERIALIZING

Or something to that effect.

--
Regards/Gruss,
Boris.

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