linux-kernel - Re: [PATCH v2 4/8] x86, lib: Add WBNOINVD helper functions

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Message-ID: <aCxORJga9goyt9N7@gmail.com>
Date: Tue, 20 May 2025 11:41:24 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, Zheyun Shen <szy0127@...u.edu.cn>,
	Tom Lendacky <thomas.lendacky@....com>,
	Kevin Loughlin <kevinloughlin@...gle.com>,
	Kai Huang <kai.huang@...el.com>, Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH v2 4/8] x86, lib: Add WBNOINVD helper functions


* Sean Christopherson <seanjc@...gle.com> wrote:

> > and to point out that the 'invalidate' part of the WBNOINVD name is 
> > a misnomer, as it doesn't invalidate anything, it only writes back 
> > dirty cachelines.
> 
> I wouldn't call it a misnomer, the NO part makes it semantically 
> accurate.

If only 'NO' in that context was unambiguous: initially I fully read it 
as some sort of acronym or abbreviation :) Why wasn't it named WBCACHE 
or so? But I digress.

> [...]  I actually think the mnemonic was well chosen, as it helps 
> capture the relationships and behaviors of INVD, WBINVD, and 
> WBNOINVD.
> 
> How about this?

Much better!

> +/*
> + * Write back all modified lines in all levels of cache associated with this
> + * logical processor to main memory, and then invalidate all caches.  Depending
> + * on the micro-architecture, WBINVD (and WBNOINVD below) may or may not affect
> + * lower level caches associated with another logical processor that shares any
> + * level of this processor’s cache hierarchy.
> + *
> + * Note, AMD CPUs enumerate the behavior or WB{NO}{INVD} with respect to other
> + * logical, non-originating processors in CPUID 0x8000001D.EAX[N:0].
> + */
>  static __always_inline void wbinvd(void)
>  {
> +       asm volatile("wbinvd" : : : "memory");
> +}
> +
> +/* Instruction encoding provided for binutils backwards compatibility. */
> +#define ASM_WBNOINVD _ASM_BYTES(0xf3,0x0f,0x09)
> +
> +/*
> + * Write back all modified lines in all levels of cache associated with this
> + * logical processor to main memory, but do NOT explicitly invalidate caches,
> + * i.e. leave all/most cache lines in the hierarchy in non-modified state.
> + */
> +static __always_inline void wbnoinvd(void)
> +{
> +       /*
> +        * Explicitly encode WBINVD if X86_FEATURE_WBNOINVD is unavailable even
> +        * though WBNOINVD is backwards compatible (it's simply WBINVD with an
> +        * ignored REP prefix), to guarantee that WBNOINVD isn't used if it
> +        * needs to be avoided for any reason.  For all supported usage in the
> +        * kernel, WBINVD is functionally a superset of WBNOINVD.
> +        */
> +       alternative("wbinvd", ASM_WBNOINVD, X86_FEATURE_WBNOINVD);
>  }

Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo