linux-kernel - Re: [PATCH v1 12/26] x86/cpu: Use scanned CPUID(0x1)

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Message-ID: <aBnHeoNq_LBY0xxj@gmail.com>
Date: Tue, 6 May 2025 10:25:30 +0200
From: Ingo Molnar <mingo@...nel.org>
To: "Ahmed S. Darwish" <darwi@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	John Ogness <john.ogness@...utronix.de>, x86@...nel.org,
	x86-cpuid@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 12/26] x86/cpu: Use scanned CPUID(0x1)


* Ahmed S. Darwish <darwi@...utronix.de> wrote:

> Use scanned CPUID(0x1) access, instead of a direct CPUID query, at early
> boot CPU detection code.
> 
> Beside the centralization benefits of the scanned CPUID API, this allows
> using the auto-generated <cpuid/leaves.h> CPUID leaf data types and their
> full C99 bitfields instead of performing ugly bitwise operations on CPUID
> register output.
> 
> Signed-off-by: Ahmed S. Darwish <darwi@...utronix.de>
> ---
>  arch/x86/kernel/cpu/common.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 59ddf6b074f2..a08340a5e6a5 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -895,6 +895,7 @@ void get_cpu_vendor(struct cpuinfo_x86 *c)
>  void cpu_detect(struct cpuinfo_x86 *c)
>  {
>  	const struct leaf_0x0_0 *l0 = cpudata_cpuid(c, 0x0);
> +	const struct leaf_0x1_0 *l1 = cpudata_cpuid(c, 0x1);
>  
>  	c->cpuid_level = l0->max_std_leaf;
>  	*(u32 *)&c->x86_vendor_id[0] = l0->cpu_vendorid_0;
> @@ -902,17 +903,14 @@ void cpu_detect(struct cpuinfo_x86 *c)
>  	*(u32 *)&c->x86_vendor_id[8] = l0->cpu_vendorid_2;
>  
>  	c->x86 = 4;
> -	/* Intel-defined flags: level 0x00000001 */
> -	if (c->cpuid_level >= 0x00000001) {
> -		u32 junk, tfms, cap0, misc;
>  
> -		cpuid(0x00000001, &tfms, &misc, &junk, &cap0);
> -		c->x86		= x86_family(tfms);
> -		c->x86_model	= x86_model(tfms);
> -		c->x86_stepping	= x86_stepping(tfms);
> +	if (l1) {
> +		c->x86		= cpuid_family(l1);
> +		c->x86_model	= cpuid_model(l1);
> +		c->x86_stepping	= l1->stepping;
>  
> -		if (cap0 & (1<<19)) {
> -			c->x86_clflush_size = ((misc >> 8) & 0xff) * 8;
> +		if (l1->clflush) {
> +			c->x86_clflush_size = l1->clflush_size * 8;
>  			c->x86_cache_alignment = c->x86_clflush_size;
>  		}

Nice patch, it really nicely demonstrates the maintainability 
advantages of the new CPUID parser.

Thanks,

	Ingo