Re: [PATCH] x86: static_cpu_has_safe: discard dynamic check after init

From: H. Peter Anvin
Date: Mon Jan 18 2016 - 18:13:46 EST

On January 18, 2016 3:05:54 PM PST, Borislav Petkov <bp@xxxxxxx> wrote:
>On Mon, Jan 18, 2016 at 11:45:18AM -0800, H. Peter Anvin wrote:
>> I think the two-byte optimization is the real issue if there is one
>> all.
>Well, we do optimize the jumps for a couple releases now, see
>recompute_jump(). So we would always get the short JMP, if made
>by how gcc lays out the code so that the target is not far away. If the
>JMP to the dynamic branch doesn't get nopped out anyway, that is.
>We have "debug-alternative" to dump exactly what happens during
>> I don't care about the inittext, and unless I'm misremembering
>> completely altinstructions also get ejected.
>Ah yes, they do.
>> So I don't personally object to killing off the unsafe variant.

The optimization has always been there, the question is how often it actually kicks in.
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