linux-kernel - Re: [PATCH v3 1/2] x86, lib: Add WBNOINVD helper functions

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Message-ID: <CAGdbjmL=+L-sQioucz6yh_1jrtDCOz1fPxXDU2eZ_HRQkbFugg@mail.gmail.com>
Date: Tue, 21 Jan 2025 17:14:16 -0800
From: Kevin Loughlin <kevinloughlin@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	seanjc@...gle.com, pbonzini@...hat.com, kirill.shutemov@...ux.intel.com, 
	kai.huang@...el.com, ubizjak@...il.com, jgross@...e.com, kvm@...r.kernel.org, 
	thomas.lendacky@....com, pgonda@...gle.com, sidtelang@...gle.com, 
	mizhang@...gle.com, rientjes@...gle.com, manalinandan@...gle.com, 
	szy0127@...u.edu.cn
Subject: Re: [PATCH v3 1/2] x86, lib: Add WBNOINVD helper functions

On Tue, Jan 21, 2025 at 4:32 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 1/21/25 16:13, Kevin Loughlin wrote:
> > +static __always_inline void wbnoinvd(void)
> > +{
> > +     alternative("wbinvd", "wbnoinvd", X86_FEATURE_WBNOINVD);
> >  }
>
> Could we please comment this a _bit_?
>
> /*
>  * Cheaper version of wbinvd(). Call when caches
>  * need to be written back but not invalidated.
>  */
> static __always_inline void wbnoinvd(void)
> {
>         /*
>          * Use the compatible but more destructuve "invalidate"
>          * variant when no-invalidate is unavailable:
>          */
>         alternative("wbinvd", "wbnoinvd", X86_FEATURE_WBNOINVD);
> }
>
> Sure, folks can read the instruction reference, but it doesn't give you
> much of the story of why you should use one over the other or why it's
> OK to call one when you ask for the other.

Yeah, good point. Incoming in v4; thanks!