linux-kernel - Re: [PATCH] x86/cpu: Start documenting what the X86_FEATURE

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Date:   Thu, 10 Nov 2022 23:27:08 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Dave Hansen <dave.hansen@...el.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/cpu: Start documenting what the X86_FEATURE_ flag
 testing macros do

On Tue, Nov 08, 2022, Borislav Petkov wrote:
> On Mon, Nov 07, 2022 at 02:13:52PM -0800, Dave Hansen wrote:
> > It seems to be mildly warning against using _static_cpu_has()
> > indiscriminately.  Should we tone that down a bit if we're recommending
> > implicit use of static_cpu_has() via cpu_feature_enabled() everywhere?
> 
> Yeah, that comment is mine AFAIR. I was thinking of simply removing
> it as part of a long-term effort of converting everything to
> cpu_feature_enabled() and hiding static_cpu_has() eventually...

What about doing the opposite and folding cpu_feature_enabled()'s build-time
functionality into static_cpu_has() _and_ boot_cpu_has(), and then dropping
cpu_feature_enabled()?  That way the tradeoffs of using the static variant are
still captured in code (cpu_feature_enabled() sounds too innocuous to my ears),
and as an added bonus even slow paths benefit from build-time disabling of features.

Hiding the use of alternatives in cpu_feature_enabled() seems like it will lead to
unnecessary code patching.