linux-kernel - Re: [patch 53/53] x86/cpu/topology: Get rid of cpuinfo::x86_max

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Date:   Thu, 14 Dec 2023 14:00:23 +0000
From:   "Zhang, Rui" <rui.zhang@...el.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "Gross, Jurgen" <jgross@...e.com>,
        "mikelley@...rosoft.com" <mikelley@...rosoft.com>,
        "arjan@...ux.intel.com" <arjan@...ux.intel.com>,
        "kprateek.nayak@....com" <kprateek.nayak@....com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kan.liang@...ux.intel.com" <kan.liang@...ux.intel.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "ray.huang@....com" <ray.huang@....com>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "Sivanich, Dimitri" <dimitri.sivanich@....com>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "Mehta, Sohil" <sohil.mehta@...el.com>,
        "andy@...radead.org" <andy@...radead.org>,
        "Tang, Feng" <feng.tang@...el.com>
Subject: Re: [patch 53/53] x86/cpu/topology: Get rid of cpuinfo::x86_max_cores


> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -566,7 +566,7 @@ static void __init build_sched_topology(
>  void set_cpu_sibling_map(int cpu)
>  {
>         bool has_smt = topology_smt_supported();
> -       bool has_mp = has_smt || boot_cpu_data.x86_max_cores > 1;
> +       bool has_mp = has_smt || topology_num_cores_per_package() >
> 1;

This patch series, especially the change here, indeed fixes a bug
observed on a platform with below cpuid.1f output,

# taskset -c 0 cpuid -1 -l 0x1f -s 0
CPU:
   V2 extended topology (0x1f):
      x2APIC ID of logical processor = 0x0 (0)
      --- level 0 ---
      level number                          = 0x0 (0)
      level type                            = thread (1)
      bit width of level & previous levels  = 0x1 (1)
      number of logical processors at level = 0x1 (1)
# taskset -c 0 cpuid -1 -l 0x1f -s 1
CPU:
      --- level 1 ---
      level number                          = 0x1 (1)
      level type                            = core (2)
      bit width of level & previous levels  = 0x3 (3)
      number of logical processors at level = 0x1 (1)
# taskset -c 0 cpuid -1 -l 0x1f -s 2
CPU:
      --- level 2 ---
      level number                          = 0x2 (2)
      level type                            = module (3)
      bit width of level & previous levels  = 0x7 (7)
      number of logical processors at level = 0x18 (24)

The bug is that, on this platform, without this patch series,
1. has_smt is false because there is indeed no HT
2. has_mp is false because
	1. core_level_siblings from CPUID.1F Core level EBX is 1
	2. current code does not parse Module type and use
core_level_siblings as the number_of_cpus_in_a_package, so
	boot_cpu_data.x86_max_cores =
		core_level_siblings / smp_num_siblings = 1

As a result, set_cpu_sibling_map() assumes this is a UP system and does
not update any sibling maps, the topology is totally broken then.

thanks,
rui