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

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Date:   Fri, 4 Aug 2023 14:04:46 +0530
From:   K Prateek Nayak <kprateek.nayak@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        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()

Hello Thomas,

On 8/4/2023 1:58 PM, Thomas Gleixner wrote:
> 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.

Ah! I see. Thank you for clarifying. I'll keep an eye out for the
next series.

--
Thanks and Regards,
Prateek