On Mon, 2018-01-15 at 14:45 +0100, Peter Zijlstra wrote: > On Fri, Jan 12, 2018 at 10:09:08AM +0000, David Woodhouse wrote: > > static_cpu_has() + asm-goto is NOT SUFFICIENT. > > > > It's still *possible* for a missed optimisation in GCC to still leave > > us with a conditional branch around the wrmsr, letting the CPU > > speculate around it too. > > OK, so GCC would have to be bloody retarded to mess this up;
Like *that's* never happened before? In corner cases where it just gets
confused and certain optimisations go out the window?
> but would something like the below work for you?
>
> The usage is like:
>
> if (static_branch_unlikely(key)) {
> arch_static_assert();
> stuff();
> }
>
> And then objtool will fail things if the first instruction into that
> branch is not immediately after a NOP/JMP patch site (on either the NOP
> or the JMP+disp side of things).
That seems reasonable; thanks. Bonus points if you can make the
arch_static_assert happen() automatically with vile tricks like
#define IF_FEATURE(ftr) if (static_cpu_has(ftr)) arch_static_assert,
So then it just becomes
IF_FEATURE(key) {
stuff();
}
There might not be a sane way to do that though. And it's OK to have to
manually annotate the call sites where this is for correctness and not
purely optimisation.
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