linux-kernel - Re: [PATCH] x86/fpu: Use consistent test for X86_FEATURE

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Date:   Wed, 3 Feb 2021 16:54:08 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        luto@...nel.org, mingo@...nel.org, tglx@...utronix.de,
        len.brown@...el.com, ravi.v.shankar@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/fpu: Use consistent test for X86_FEATURE_XSAVES

On Wed, Feb 03, 2021 at 07:40:07AM -0800, Dave Hansen wrote:
> On 2/3/21 3:23 AM, Borislav Petkov wrote:
> >> -/*
> >> - * 'XSAVES' implies two different things:
> >> - * 1. saving of supervisor/system state
> >> - * 2. using the compacted format
> >> - *
> >> - * Use this function when dealing with the compacted format so
> >> - * that it is obvious which aspect of 'XSAVES' is being handled
> >> - * by the calling code.
> > @dhansen, are you still hung up on that "obvious aspect" or can we kill
> > this?
> 
> I still want the compacted-format handling code to be marked.  You can
> do that with new comments:
> 
> 	/* Note: XSAVES always uses compacted format: */
> 	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
> 
> or, leave it as-is:
> 
> 	if (using_compacted_format()) {
> 	...
> 
> Otherwise, we assume that every human being that looks at this code
> *KNOWS* that XSAVES==compacted.  That's not a great assumption.

Well, the reason why I reacted to this is because I was looking at
using_compacted_format() - aha this, checks X86_FEATURE_XSAVES - but
then other code paths in fpu/ check X86_FEATURE_XSAVES directly. And
this is confusing, making me wonder why is that special oneliner there.
Sure, the comment above it says why...

I guess if you wanna keep it, then we need another oneliner for 1. or
really do comments at each call site.

Hmm.

-- 
Regards/Gruss,
    Boris.

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