linux-kernel - Re: [PATCH v4 1/2] x86, lib: Add WBNOINVD helper functions

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Message-ID: <CAGdbjm+syon_W0W_oEiDJBKu4s5q9JS9cKyPmPoqDAzeyMJf3Q@mail.gmail.com>
Date: Wed, 22 Jan 2025 16:06:13 -0800
From: Kevin Loughlin <kevinloughlin@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Tom Lendacky <thomas.lendacky@....com>, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 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, kai.huang@...el.com, ubizjak@...il.com, jgross@...e.com, 
	kvm@...r.kernel.org, pgonda@...gle.com, sidtelang@...gle.com, 
	mizhang@...gle.com, rientjes@...gle.com, manalinandan@...gle.com, 
	szy0127@...u.edu.cn
Subject: Re: [PATCH v4 1/2] x86, lib: Add WBNOINVD helper functions

On Wed, Jan 22, 2025 at 3:16 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 1/22/25 11:39, Tom Lendacky wrote:
> >> I think you need to do something like.
> >>
> >>      alternative("wbinvd", ".byte 0xf3; wbinvd", X86_FEATURE_WBNOINVD);
> > I think "rep; wbinvd" would work as well.
>
> I don't want to bike shed this too much.
>
> But, please, no.
>
> The fact that wbnoinvd can be expressed as "rep; wbinvd" is a
> implementation detail. Exposing how it's encoded in the ISA is only
> going to add confusion. This:
>
> static __always_inline void wbnoinvd(void)
> {
>         asm volatile(".byte 0xf3,0x0f,0x09\n\t": : :"memory");
> }
>
> is perfectly fine and a billion times less confusing than:
>
>         asm volatile(".byte 0xf3; wbinvd\n\t": : :"memory");
> or
>         asm volatile("rep; wbinvd\n\t": : :"memory");
>
> It only gets worse if it's mixed in an alternative() with the _actual_
> wbinvd.

Works for me. I will explicitly use 0xf3,0x0f,0x09 in v5, which I will
send shortly.

> BTW, I don't think you should be compelled to use alternative() as
> opposed to a good old:
>
>         if (cpu_feature_enabled(X86_FEATURE_WBNOINVD))
>                 ...

Agreed, though I'm leaving as alternative() for now (both because it
results in fewer checks and because that's what is used in the rest of
the file); please holler if you prefer otherwise. If so, my slight
preference in that case would be to update the whole file
stylistically in a separate commit.