linux-kernel - A question about prefetchw detection in "x86/asm: Cleanup prefetch primitives"

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Date:   Wed, 21 Jun 2023 18:57:40 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>
Subject: A question about prefetchw detection in "x86/asm: Cleanup prefetch
 primitives"

Hi,

In commit a930dc4543a2 ("x86/asm: Cleanup prefetch primitives"), a chunk
of code was added:

	/* 3DNow or LM implies PREFETCHW */
	if (!cpu_has(c, X86_FEATURE_3DNOWPREFETCH))
		if (cpu_has(c, X86_FEATURE_3DNOW) || cpu_has(c, X86_FEATURE_LM))
			set_cpu_cap(c, X86_FEATURE_3DNOWPREFETCH);

But Glibc does not contain a similar logic in the CPU feature detection
code, causing a test failure on old CPUs
(https://sourceware.org/bugzilla/show_bug.cgi?id=30328).

I intend to implement the same logic for Glibc.  I can understand "3DNow
implies PREFETCHW", but is there a bibliographical reference about "LM
implies PREFETCHW" so I can convince the Glibc maintainers for the
change?

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University