linux-kernel - Re: [PATCH v4 2/2] x86/amd: Fixup cpu_core_id for family17h downcore configuration

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Date:   Fri, 28 Jul 2017 11:39:43 +0200
From:   Andreas Herrmann <aherrmann@...e.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, peterz@...radead.org,
        Yazen.Ghannam@....com
Subject: Re: [PATCH v4 2/2] x86/amd: Fixup cpu_core_id for family17h downcore
 configuration

On Mon, Jul 24, 2017 at 04:44:45PM +0200, Borislav Petkov wrote:
> On Mon, Jul 24, 2017 at 09:14:18PM +0700, Suravee Suthikulpanit wrote:
> > Actually, this is not totally accurate. My apology. This patch is
> > mainly fix to incorrect core ID in /proc/cpuinfo.
> 
> So you're "fixing" only some numbering thing. Because core_id doesn't
> have any influence on anything. Here's on an Intel box I have here:
> 
> processor :  0		 physical id : 0	 core id : 0
> processor :  1		 physical id : 1	 core id : 0

  ---8<---

> processor : 62		 physical id : 2	 core id : 11
> processor : 63		 physical id : 3	 core id : 11
> 
> So those core id numbers can be anything as long as the cpumasks used by
> the scheduler are correct.

And as long as the user is able to find consistent topology
information.  (Ie. via /proc/cpuinfo and/or
/sys/bus/cpu/devices/cpuX/topology/)

> > This is due to the cpu_core_id fixup in amd_get_topology() below:
> > 
> >         /* fixup multi-node processor information */
> >         if (nodes_per_socket > 1) {
> >                 u32 cus_per_node;
> > 
> >                 set_cpu_cap(c, X86_FEATURE_AMD_DCM);
> >                 cus_per_node = c->x86_max_cores / nodes_per_socket;
> > 
> >                 /* core id has to be in the [0 .. cores_per_node - 1] range */
> >                 c->cpu_core_id %= cus_per_node;
> >         }
> 
> AFAICT, Andreas did this for MC at the time:
> 
> 4a376ec3a259 ("x86: Fix CPU llc_shared_map information for AMD Magny-Cours")
> 
> but I don't think we need to care about core_ids fitting into the node
> range anymore. For the above reason - topology doesn't use core ids.

IIRC this was the way to fix llc_shared_map as CPUID functions could
not be used to figure which cores belong to the same compute unit.

> So you can just as well let ->cpu_core_id be derived from the
> ->initial_apicid as it is being done now in amd_detect_cmp().
> 
> In order not to cause any more confusion, you can limit the above fixup
> to anything below F17h so that we don't upset existing users and add a
> big fat comment as to why we're doing this. But if it is only a silly
> numbering thing, I don't see the need for doing that jumping through
> hoops.
> 
> -- 
> Regards/Gruss,
>     Boris.

Regards,

Andreas