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

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Date: Mon, 12 Feb 2024 15:17:45 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Borislav Petkov <bp@...en8.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 30 2024 at 20:31, Borislav Petkov wrote:
> 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?

TBH, the // comment style is really better for struct definitions. It's
denser and easier to parse.

		// eax
		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

versus:

		/* eax */
		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 */

Especially x2apic_shift is horrible and the comments of EBX are visually
impaired while with the C++ comments x2apic_shift looks natural and the
EBX comments are just open to the right and therefore simpler.

>> +	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...?

If it's caught in early testing, this should be fixed, no?

Thanks,

        tglx