Re: [PATCH v5 28/28] x86/fpu/amx: Clear the AMX state when appropriate

[Len Brown]

On Mon, May 24, 2021 at 1:39 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:

> >> might be considered nasty.

> > I'm not excited about burdening the generic idle path with a CPU
> > feature specific check that would need to be checked on every idle
> > entry.
>
> Me neither.
>
> But, the check itself should be cheap.  A cpu_feature_enabled(AMX) check
> will eliminate even the cost of a branch on systems without AMX.  You
> could probably even get fancy and also use a static branch that doesn't
> get enabled until the first AMX user shows up.

It isn't just the hardware run-time cost.
It is the source code complexity.
That code is 100% generic.

Len Brown, Intel Open Source Technology Center