File 0001-x86-work-around-legacy_max_proc-being-0-while-HTT-fe.patch of Package hwloc

From 77495cecad7178ccd73ad4962780328f079a0e65 Mon Sep 17 00:00:00 2001
From: Brice Goglin <Brice.Goglin@inria.fr>
Date: Thu, 24 Apr 2025 09:08:08 +0200
Subject: [PATCH] x86: work around legacy_max_proc being 0 while HTT feature
 bit is set

The Intel manual says that legacy_max_proc (CPUID.1.EBX[16-23]) is valid
if CPUID.1.EDX.HTT[bit 28] is set.
AMD (at least recent ones) don't say anything about it being invalid.

Unfortunately some Qemu config may keep the former at 0 with the latter set.
At least this happens when libvirt passes -cpu EPYC-Rome,ht=on to Qemu
(which sets the HTT bit), and -smp 32,maxcpus=48,sockets=48,cores=1,threads=1
says each CPU is single threaded (which keeps legacy_max_log_proc to 0).
This config comes from https://bugzilla.opensuse.org/show_bug.cgi?id=1236038

Calling flsl on this invalid mask leads to undefined behavior and some division
by zero later (depending on the compiler).

Check whether legacy_max_proc is 0 before using it.
If 0, assume legacy_max_log_proc is 1, just like we did when HTT is unset.

Thanks to Georg Pfuetzenreuter for the report
and Anthony Iliopoulos for the debugging.

Refs #714

Signed-off-by: Brice Goglin <Brice.Goglin@inria.fr>
---
 hwloc/topology-x86.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/hwloc/topology-x86.c b/hwloc/topology-x86.c
index a267ded49..5f63fc178 100644
--- a/hwloc/topology-x86.c
+++ b/hwloc/topology-x86.c
@@ -653,7 +653,13 @@ static void look_proc(struct hwloc_backend *backend, struct procinfo *infos, uns
   cpuid_or_from_dump(&eax, &ebx, &ecx, &edx, src_cpuiddump);
   infos->apicid = ebx >> 24;
   if (edx & (1 << 28)) {
-    legacy_max_log_proc = 1 << hwloc_flsl(((ebx >> 16) & 0xff) - 1);
+    unsigned ebx_16_23 = (ebx >> 16) & 0xff;
+    if (ebx_16_23) {
+      legacy_max_log_proc = 1 << hwloc_flsl(ebx_16_23 - 1);
+    } else {
+      hwloc_debug("HTT bit set in CPUID 0x01.edx, but legacy_max_proc = 0 in ebx, assuming legacy_max_log_proc = 1\n");
+      legacy_max_log_proc = 1;
+    }
   } else {
     hwloc_debug("HTT bit not set in CPUID 0x01.edx, assuming legacy_max_log_proc = 1\n");
     legacy_max_log_proc = 1;
-- 
2.48.1