linux-kernel - Re: [patch V3 23/40] x86/cpu: Provide cpu_init/parse

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Date:   Fri, 04 Aug 2023 10:28:30 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     K Prateek Nayak <kprateek.nayak@....com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     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>,
        Michael Kelley <mikelley@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>
Subject: Re: [patch V3 23/40] x86/cpu: Provide cpu_init/parse_topology()

On Fri, Aug 04 2023 at 13:44, K Prateek Nayak wrote:
> On 8/2/2023 3:51 PM, Thomas Gleixner wrote:
>>
>> [..snip..]
>>
>> +static void topo_set_max_cores(struct topo_scan *tscan)
>> +{
>> +	/*
>> +	 * Bug compatible for now. This is broken on hybrid systems:
>> +	 * 8 cores SMT + 8 cores w/o SMT
>> +	 * tscan.dom_ncpus[TOPO_CORE_DOMAIN] = 24; 24 / 2 = 12 !!
>> +	 *
>> +	 * Cannot be fixed without further topology enumeration changes.
>> +	 */
>> +	tscan->c->x86_max_cores = tscan->dom_ncpus[TOPO_CORE_DOMAIN] >>
>> +		x86_topo_system.dom_shifts[TOPO_SMT_DOMAIN];
>> +}
>>
>
> In Documentation/arch/x86/topology.rst, "cpuinfo_x86.x86_max_cores" is
> described as "The number of cores in a package". In which case,
> shouldn't the above be:
>
> 	tscan->c->x86_max_cores = tscan->dom_ncpus[TOPO_PKG_DOMAIN] >>
> 		x86_topo_system.dom_shifts[TOPO_SMT_DOMAIN];
>
> since, with extended topology, there could be other higher domains and
> dom_ncpus[TOPO_CORE_DOMAIN] >> dom_shifts[TOPO_SMT_DOMAIN] should only
> give number of cores within the next domain (TOPO_MODULE_DOMAIN).

You're right in principle.

> Am I missing something here?

The fact, that this is bug compatible. It's broken in several
aspects. The real fix is in the next series, where this function goes
away and actually uses real topology data to compute this.

I could change this to be more "accurate" as you suggested, but that's
not making much of a difference.

Thanks,

        tglx