linux-kernel - Re: [PATCH v4 28/34] x86/cacheinfo: Use parsed CPUID(0x80000005) and CPUID(0x80000006)

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Message-ID: <aKNJB1uM4LIX3WDw@lx-t490>
Date: Mon, 18 Aug 2025 17:38:47 +0200
From: "Ahmed S. Darwish" <darwi@...utronix.de>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Sohil Mehta <sohil.mehta@...el.com>,
	John Ogness <john.ogness@...utronix.de>, x86@...nel.org,
	x86-cpuid@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 28/34] x86/cacheinfo: Use parsed CPUID(0x80000005) and
 CPUID(0x80000006)

On Fri, 15 Aug 2025, Dave Hansen wrote:
>
> On 8/15/25 00:02, Ahmed S. Darwish wrote:
> > +static void legacy_amd_cpuid4(struct cpuinfo_x86 *c, int index, struct leaf_0x4_0 *regs)
> >  {
> > -	unsigned int dummy, line_size, lines_per_tag, assoc, size_in_kb;
> > -	union l1_cache l1i, l1d, *l1;
> > -	union l2_cache l2;
> > -	union l3_cache l3;
> > +	const struct leaf_0x80000005_0 *el5 = cpuid_leaf(c, 0x80000005);
> > +	const struct leaf_0x80000006_0 *el6 = cpuid_leaf(c, 0x80000006);
> > +	const struct cpuid_regs *el5_raw = (const struct cpuid_regs *)el5;
>
> Is there any way we could get rid of the casts? The lack of type safety
> on those always worries me. Maybe a helper like this:
>
>     const struct cpuid_regs *el5_raw = cpuid_leaf_raw(c, 0x80000006);
>
> (although that would probably just do casts internally).
>

Indeed.

I already have at <asm/cpuid/api.h>:

    /**
     * cpuid_leaf_regs() - Access parsed CPUID data in raw format
     * @_cpuinfo:	CPU capability structure reference ('struct cpuinfo_x86')
     * @_leaf:		CPUID leaf, in compile-time 0xN format
     *
     * Similar to cpuid_leaf(), but returns a raw 'struct cpuid_regs' pointer to
     * the parsed CPUID data instead of a "typed" <cpuid/leaf_types.h> pointer.
     */
    #define cpuid_leaf_regs(_cpuinfo, _leaf)				\
	((struct cpuid_regs *)(cpuid_leaf(_cpuinfo, _leaf)))

The only reason I didn't use it in this patch was to avoid a repetion.
But, you're correct, using the API is better than call-site casting.

Thus, the code shall be:

    const struct leaf_0x80000005_0 *el5 = cpuid_leaf(c, 0x80000005);
    const struct leaf_0x80000006_0 *el6 = cpuid_leaf(c, 0x80000006);
    const struct cpuid_regs *el5_raw = cpuid_leaf_regs(c, 0x80000005);

I'll do it like that in v5.

Thanks!
Ahmed