[PATCH 1/1] x86/topology: fix erroneous smp_num_siblings on Intel Hybrid platform

[Zhang Rui]

The SMT siblings value returned by CPUID.1F SMT level EBX differs
among CPUs on Intel Hybrid platforms like AlderLake and MeteorLake.
It returns 2 for Pcore CPUs which have SMT siblings and returns 1 for
Ecore CPUs which do not have SMT siblings.

Today, the CPU boot code sets the global variable smp_num_siblings when
every CPU thread is brought up. The last thread to boot will overwrite
it with the number of siblings of *that* thread. That last thread to
boot will "win". If the thread is a Pcore, smp_num_siblings == 2.  If it
is an Ecore, smp_num_siblings == 1.

smp_num_siblings describes if the *system* supports SMT.  It should
specify the maximum number of SMT threads among all cores.

On AlderLake-P/S platforms, it does not cause any functional issues so
far.
But on MeteorLake-P platform, when probing an Ecore CPU,
a). smp_num_siblings varies like AlderLake and it is set to 1 for Ecore.
b). x86_max_cores is totally broken and it is set to 1 for the boot cpu.
Altogether, these two issues make the system being treated as an UP
system in set_cpu_sibling_map() when probing Ecore CPUs, and the Ecore
CPUs are not updated in any cpu sibling maps erroneously.

Below shows part of the CPU topology information before and after the
fix, for both Pcore and Ecore CPU (cpu0 is Pcore, cpu 12 is Ecore).
...
-/sys/devices/system/cpu/cpu0/topology/package_cpus:000fff
-/sys/devices/system/cpu/cpu0/topology/package_cpus_list:0-11
+/sys/devices/system/cpu/cpu0/topology/package_cpus:3fffff
+/sys/devices/system/cpu/cpu0/topology/package_cpus_list:0-21
...
-/sys/devices/system/cpu/cpu12/topology/package_cpus:001000
-/sys/devices/system/cpu/cpu12/topology/package_cpus_list:12
+/sys/devices/system/cpu/cpu12/topology/package_cpus:3fffff
+/sys/devices/system/cpu/cpu12/topology/package_cpus_list:0-21

And this also breaks userspace tools like lscpu
-Core(s) per socket:  1
-Socket(s):           11
+Core(s) per socket:  16
+Socket(s):           1

To fix the first issue, ensure that smp_num_siblings represents the
system-wide maximum number of siblings by always increasing its value.
Never allow it to decrease.

Note that this fix is sufficient to make set_cpu_sibling_map() work
correctly. And how to fix the bogus cpuinfo_x86.x86_max_cores will be
addressed separately.

CC: stable@xxxxxxxxxx
Suggested-by: Len Brown <len.brown@xxxxxxxxx>
Signed-off-by: Zhang Rui <rui.zhang@xxxxxxxxx>
---
 arch/x86/kernel/cpu/topology.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
index 5e868b62a7c4..0270925fe013 100644
--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -79,7 +79,7 @@ int detect_extended_topology_early(struct cpuinfo_x86 *c)
 	 * initial apic id, which also represents 32-bit extended x2apic id.
 	 */
 	c->initial_apicid = edx;
-	smp_num_siblings = LEVEL_MAX_SIBLINGS(ebx);
+	smp_num_siblings = max_t(int, smp_num_siblings, LEVEL_MAX_SIBLINGS(ebx));
 #endif
 	return 0;
 }
@@ -109,7 +109,8 @@ int detect_extended_topology(struct cpuinfo_x86 *c)
 	 */
 	cpuid_count(leaf, SMT_LEVEL, &eax, &ebx, &ecx, &edx);
 	c->initial_apicid = edx;
-	core_level_siblings = smp_num_siblings = LEVEL_MAX_SIBLINGS(ebx);
+	core_level_siblings = LEVEL_MAX_SIBLINGS(ebx);
+	smp_num_siblings = max_t(int, smp_num_siblings, LEVEL_MAX_SIBLINGS(ebx));
 	core_plus_mask_width = ht_mask_width = BITS_SHIFT_NEXT_LEVEL(eax);
 	die_level_siblings = LEVEL_MAX_SIBLINGS(ebx);
 	pkg_mask_width = die_plus_mask_width = BITS_SHIFT_NEXT_LEVEL(eax);
-- 
2.25.1