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Subject[PATCH] x86, sched: Allow NUMA nodes to share an LLC on Intel platforms
Date
Commit 1340ccfa9a9a ("x86,sched: Allow topologies where NUMA nodes
share an LLC") added a vendor and model specific check to skip the
topology_sane() check for Intel's Sky Lake Server CPUs where NUMA
nodes shared an LLC.

This topology is no longer a quirk for Intel CPUs as Ice Lake and
Sapphire Rapids CPUs exhibit the same topology. Rather than maintain
the quirk list, define a synthetic flag that directs the scheduler
to allow this topology without warning for all Intel CPUs when NUMA
is configured.

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: brice.goglin@gmail.com
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/kernel/cpu/intel.c | 15 +++++++++++++++
arch/x86/kernel/smpboot.c | 23 ++---------------------
3 files changed, 18 insertions(+), 21 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 84b887825f12..bec74b90d3d6 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -421,5 +421,6 @@
#define X86_BUG_TAA X86_BUG(22) /* CPU is affected by TSX Async Abort(TAA) */
#define X86_BUG_ITLB_MULTIHIT X86_BUG(23) /* CPU may incur MCE during certain page attribute changes */
#define X86_BUG_SRBDS X86_BUG(24) /* CPU may leak RNG bits if not mitigated */
+#define X86_BUG_NUMA_SHARES_LLC X86_BUG(25) /* CPU may enumerate an LLC shared by multiple NUMA nodes */

#endif /* _ASM_X86_CPUFEATURES_H */
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 816fdbec795a..027348261080 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -719,6 +719,21 @@ static void init_intel(struct cpuinfo_x86 *c)
tsx_disable();

split_lock_init();
+
+ /*
+ * Set X86_BUG_NUMA_SHARES_LLC to allow topologies where NUMA
+ * nodes share an LLC. In Sub-NUMA Clustering mode Intel CPUs
+ * may enumerate an LLC as shared by multiple NUMA nodes. The
+ * LLC is shared for off-package data access but private to
+ * the NUMA node for on-package access. This topology first
+ * appeared in SKYLAKE_X. It was treated as a quirk and allowed.
+ * This topology reappeared in ICELAKE_X and SAPPHIRERAPIDS_X.
+ * Rather than maintain a list of quirk CPUS, allow this topology
+ * on all Intel CPUs with NUMA configured. When this X86_BUG is
+ * set, the scheduler accepts this topology without warning.
+ */
+ if (IS_ENABLED(CONFIG_NUMA))
+ set_cpu_bug(c, X86_BUG_NUMA_SHARES_LLC);
}

#ifdef CONFIG_X86_32
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 117e24fbfd8a..7d05c3552795 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -458,26 +458,6 @@ static bool match_smt(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
return false;
}

-/*
- * Define snc_cpu[] for SNC (Sub-NUMA Cluster) CPUs.
- *
- * These are Intel CPUs that enumerate an LLC that is shared by
- * multiple NUMA nodes. The LLC on these systems is shared for
- * off-package data access but private to the NUMA node (half
- * of the package) for on-package access.
- *
- * CPUID (the source of the information about the LLC) can only
- * enumerate the cache as being shared *or* unshared, but not
- * this particular configuration. The CPU in this case enumerates
- * the cache to be shared across the entire package (spanning both
- * NUMA nodes).
- */
-
-static const struct x86_cpu_id snc_cpu[] = {
- X86_MATCH_INTEL_FAM6_MODEL(SKYLAKE_X, NULL),
- {}
-};
-
static bool match_llc(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
{
int cpu1 = c->cpu_index, cpu2 = o->cpu_index;
@@ -495,7 +475,8 @@ static bool match_llc(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
* means 'c' does not share the LLC of 'o'. This will be
* reflected to userspace.
*/
- if (!topology_same_node(c, o) && x86_match_cpu(snc_cpu))
+ if (!topology_same_node(c, o) &&
+ boot_cpu_has_bug(X86_BUG_NUMA_SHARES_LLC))
return false;

return topology_sane(c, o, "llc");
--
2.20.1
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