linux-kernel - Re: [patch v5 06/19] x86/cpu: Provide a sane leaf 0xb/0x1f parser

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Message-ID: <20240130193102.GEZblOdor_bzoVhT0f@fat_crate.local>
Date: Tue, 30 Jan 2024 20:31:02 +0100
From: Borislav Petkov <bp@...en8.de>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	Tom Lendacky <thomas.lendacky@....com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Huang Rui <ray.huang@....com>, Juergen Gross <jgross@...e.com>,
	Dimitri Sivanich <dimitri.sivanich@....com>,
	Sohil Mehta <sohil.mehta@...el.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Kan Liang <kan.liang@...ux.intel.com>,
	Zhang Rui <rui.zhang@...el.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Feng Tang <feng.tang@...el.com>,
	Andy Shevchenko <andy@...radead.org>,
	Michael Kelley <mhklinux@...look.com>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [patch v5 06/19] x86/cpu: Provide a sane leaf 0xb/0x1f parser

On Tue, Jan 23, 2024 at 01:53:39PM +0100, Thomas Gleixner wrote:
> +static inline bool topo_subleaf(struct topo_scan *tscan, u32 leaf, u32 subleaf,

"parse_topo_subleaf"?

With a verb in the name...

> +				unsigned int *last_dom)
> +{
> +	unsigned int dom, maxtype;
> +	const unsigned int *map;
> +	struct {
> +		// eax

Can we please not use those yucky // comments together with the
multiline ones?

> +		u32	x2apic_shift	:  5, // Number of bits to shift APIC ID right
> +					      // for the topology ID at the next level
> +					: 27; // Reserved
> +		// ebx
> +		u32	num_processors	: 16, // Number of processors at current level
> +					: 16; // Reserved
> +		// ecx
> +		u32	level		:  8, // Current topology level. Same as sub leaf number
> +			type		:  8, // Level type. If 0, invalid
> +					: 16; // Reserved
> +		// edx
> +		u32	x2apic_id	: 32; // X2APIC ID of the current logical processor
> +	} sl;

..

> +static bool parse_topology_leaf(struct topo_scan *tscan, u32 leaf)
> +{
> +	unsigned int last_dom;
> +	u32 subleaf;
> +
> +	/* Read all available subleafs and populate the levels */
> +	for (subleaf = 0, last_dom = 0; topo_subleaf(tscan, leaf, subleaf, &last_dom); subleaf++);
> +
> +	/* If subleaf 0 failed to parse, give up */
> +	if (!subleaf)
> +		return false;
> +
> +	/*
> +	 * There are machines in the wild which have shift 0 in the subleaf
> +	 * 0, but advertise 2 logical processors at that level. They are
> +	 * truly SMT.
> +	 */
> +	if (!tscan->dom_shifts[TOPO_SMT_DOMAIN] && tscan->dom_ncpus[TOPO_SMT_DOMAIN] > 1) {
> +		unsigned int sft = get_count_order(tscan->dom_ncpus[TOPO_SMT_DOMAIN]);
> +
> +		pr_warn_once(FW_BUG "CPUID leaf 0x%x subleaf 0 has shift level 0 but %u CPUs\n",
> +			     leaf, tscan->dom_ncpus[TOPO_SMT_DOMAIN]);

Do you really wanna warn about that? Hoping that someone would do
something about it while there's time...?

> +		topology_update_dom(tscan, TOPO_SMT_DOMAIN, sft, tscan->dom_ncpus[TOPO_SMT_DOMAIN]);
> +	}
> +
> +	set_cpu_cap(tscan->c, X86_FEATURE_XTOPOLOGY);
> +	return true;
> +}

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette