[PATCH RFC 0/3] riscv: Add explicit die-level CPU topology

[PATCH RFC 0/3] riscv: Add explicit die-level CPU topology

[Troy Mitchell]

The generic arch_topology model has no die level. Systems with multiple
dies in one package must flatten their clusters or report each die as a
separate package, losing the die or package boundary.

This RFC adds a die ID and sibling mask to arch_topology, parses optional
dieN nodes in DT cpu-map, and exposes die topology through sysfs on RISC-V.
Clusters can be numbered independently within each die.

The RISC-V sysfs behavior is:

  - An explicit dieN node supplies a non-negative die ID within its package
    and a die sibling mask. RISC-V exposes die_id, die_cpus, and
    die_cpus_list.
  - A DT without dieN keeps die_id at -1 and creates no die sysfs files.
    Package and cluster parsing is unchanged.
  - ACPI die discovery is not implemented. The ACPI and fallback paths
    also leave die_id at -1 and create no die sysfs files.

The DT parser is shared by arm64 and RISC-V, so both record dieN nodes
internally. Only RISC-V exposes the die sysfs files in this series. If you
think I should enable this for arm64 as well, please let me know.

The sysfs visibility hook defaults to visible on other architectures,
including powerpc configurations that report die_id as -1. Their existing
sysfs behavior is unchanged.

The dieN hierarchy is a proposed extension to the DT cpu-map binding.
This RFC does not include a schema update. I'd like feedback on the
topology model and sysfs interface before sending the binding update.

Does placing dieN between socket and cluster make sense? I'd also
appreciate feedback on keeping unspecified dies at -1 and on the sysfs
visibility hook.

Tested with W=1 builds of the affected objects for RISC-V, arm64, and ARM,
and with a complete RISC-V Image build. On a 32-CPU QEMU virt guest, a DT
describing one package with two dies, four clusters per die, and four
cores per cluster exposed the expected IDs and masks and updated them
across CPU offline/online. The same guest with a legacy DT exposed no die
sysfs files while retaining package and cluster topology.

Signed-off-by: Troy Mitchell <troy.mitchell@linux.dev>
---
Troy Mitchell (3):
      arch_topology: Add a die level to CPU topology
      arch_topology: Parse die nodes in /cpu-map
      riscv: topology: Expose explicit CPU die topology

 arch/riscv/include/asm/topology.h |  4 +++
 drivers/base/arch_topology.c      | 68 +++++++++++++++++++++++++++++++--------
 drivers/base/topology.c           | 28 +++++++++++++++-
 include/linux/arch_topology.h     |  2 ++
 include/linux/topology.h          |  3 ++
 5 files changed, 91 insertions(+), 14 deletions(-)
---
base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
change-id: 20260904-riscv-die-topology-rfc-c009f6e4be92

Best regards,
--  
Troy Mitchell <troy.mitchell@linux.dev>

[PATCH RFC 1/3] arch_topology: Add a die level to CPU topology

[Troy Mitchell]

The generic topology model records packages and clusters but has no
representation for a die between them. Systems with multiple dies in one
package must therefore either report each die as a package or flatten
clusters across all dies. Both choices lose part of the physical hierarchy.

Add a die identifier and sibling mask between the package and cluster
levels. Initialize the identifier to -1 and only build die sibling masks
when firmware supplied a valid identifier. CPU topology paths without die
information, including the current ACPI parser and the fallback topology,
therefore keep the die unknown instead of inventing die 0.

Scope cluster sibling matching to a die so each die can number its
clusters independently, and maintain the new mask across CPU hotplug.

Signed-off-by: Troy Mitchell <troy.mitchell@linux.dev>
---
 drivers/base/arch_topology.c  | 20 +++++++++++++++++---
 include/linux/arch_topology.h |  2 ++
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 8c5e47c28d9a3..ee82827be3351 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -766,6 +766,14 @@ void update_siblings_masks(unsigned int cpuid)
 		cpumask_set_cpu(cpuid, &cpu_topo->core_sibling);
 		cpumask_set_cpu(cpu, &cpuid_topo->core_sibling);
 
+		if (cpuid_topo->die_id != cpu_topo->die_id)
+			continue;
+
+		if (cpuid_topo->die_id >= 0) {
+			cpumask_set_cpu(cpu, &cpuid_topo->die_sibling);
+			cpumask_set_cpu(cpuid, &cpu_topo->die_sibling);
+		}
+
 		if (cpuid_topo->cluster_id != cpu_topo->cluster_id)
 			continue;
 
@@ -792,6 +800,9 @@ static void clear_cpu_topology(int cpu)
 	cpumask_clear(&cpu_topo->cluster_sibling);
 	cpumask_set_cpu(cpu, &cpu_topo->cluster_sibling);
 
+	cpumask_clear(&cpu_topo->die_sibling);
+	cpumask_set_cpu(cpu, &cpu_topo->die_sibling);
+
 	cpumask_clear(&cpu_topo->core_sibling);
 	cpumask_set_cpu(cpu, &cpu_topo->core_sibling);
 	cpumask_clear(&cpu_topo->thread_sibling);
@@ -808,6 +819,7 @@ void __init reset_cpu_topology(void)
 		cpu_topo->thread_id = -1;
 		cpu_topo->core_id = -1;
 		cpu_topo->cluster_id = -1;
+		cpu_topo->die_id = -1;
 		cpu_topo->package_id = -1;
 
 		clear_cpu_topology(cpu);
@@ -824,6 +836,8 @@ void remove_cpu_topology(unsigned int cpu)
 		cpumask_clear_cpu(cpu, topology_sibling_cpumask(sibling));
 	for_each_cpu(sibling, topology_cluster_cpumask(cpu))
 		cpumask_clear_cpu(cpu, topology_cluster_cpumask(sibling));
+	for_each_cpu(sibling, &cpu_topology[cpu].die_sibling)
+		cpumask_clear_cpu(cpu, &cpu_topology[sibling].die_sibling);
 	for_each_cpu(sibling, topology_llc_cpumask(cpu))
 		cpumask_clear_cpu(cpu, topology_llc_cpumask(sibling));
 
@@ -969,9 +983,9 @@ void store_cpu_topology(unsigned int cpuid)
 	cpuid_topo->core_id = cpuid;
 	cpuid_topo->package_id = cpu_to_node(cpuid);
 
-	pr_debug("CPU%u: package %d core %d thread %d\n",
-		 cpuid, cpuid_topo->package_id, cpuid_topo->core_id,
-		 cpuid_topo->thread_id);
+	pr_debug("CPU%u: package %d die %d core %d thread %d\n",
+		 cpuid, cpuid_topo->package_id, cpuid_topo->die_id,
+		 cpuid_topo->core_id, cpuid_topo->thread_id);
 
 topology_populated:
 	update_siblings_masks(cpuid);
diff --git a/include/linux/arch_topology.h b/include/linux/arch_topology.h
index ebd7f8935f969..2a5591bb95075 100644
--- a/include/linux/arch_topology.h
+++ b/include/linux/arch_topology.h
@@ -63,10 +63,12 @@ struct cpu_topology {
 	int thread_id;
 	int core_id;
 	int cluster_id;
+	int die_id;
 	int package_id;
 	cpumask_t thread_sibling;
 	cpumask_t core_sibling;
 	cpumask_t cluster_sibling;
+	cpumask_t die_sibling;
 	cpumask_t llc_sibling;
 };
 

-- 
2.55.0

[PATCH RFC 2/3] arch_topology: Parse die nodes in /cpu-map

[Troy Mitchell]

The devicetree CPU map can distinguish packages and clusters but cannot
describe multiple dies in one package. Such systems have to flatten all
clusters into the package, making cluster identifiers package-wide and
hiding the die boundary.

Recognize an optional sequence of dieN nodes between a socket and its
clusters, and propagate the die identifier to every CPU below it. Enumerate
clusters independently below each die.

When a CPU map has no dieN nodes, continue parsing its existing package
and cluster hierarchy but pass -1 as the die identifier. Legacy maps
therefore remain compatible without being reported as an explicitly
described die 0.

Signed-off-by: Troy Mitchell <troy.mitchell@linux.dev>
---
 drivers/base/arch_topology.c | 48 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 38 insertions(+), 10 deletions(-)

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index ee82827be3351..67a984f3ba8d9 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c
@@ -498,7 +498,7 @@ static int __init get_cpu_for_node(struct device_node *node)
 }
 
 static int __init parse_core(struct device_node *core, int package_id,
-			     int cluster_id, int core_id)
+			     int die_id, int cluster_id, int core_id)
 {
 	char name[20];
 	bool leaf = true;
@@ -517,6 +517,7 @@ static int __init parse_core(struct device_node *core, int package_id,
 		cpu = get_cpu_for_node(t);
 		if (cpu >= 0) {
 			cpu_topology[cpu].package_id = package_id;
+			cpu_topology[cpu].die_id = die_id;
 			cpu_topology[cpu].cluster_id = cluster_id;
 			cpu_topology[cpu].core_id = core_id;
 			cpu_topology[cpu].thread_id = i;
@@ -538,6 +539,7 @@ static int __init parse_core(struct device_node *core, int package_id,
 		}
 
 		cpu_topology[cpu].package_id = package_id;
+		cpu_topology[cpu].die_id = die_id;
 		cpu_topology[cpu].cluster_id = cluster_id;
 		cpu_topology[cpu].core_id = core_id;
 	} else if (leaf && cpu != -ENODEV) {
@@ -549,7 +551,7 @@ static int __init parse_core(struct device_node *core, int package_id,
 }
 
 static int __init parse_cluster(struct device_node *cluster, int package_id,
-				int cluster_id, int depth)
+				int die_id, int cluster_id, int depth)
 {
 	char name[20];
 	bool leaf = true;
@@ -572,7 +574,7 @@ static int __init parse_cluster(struct device_node *cluster, int package_id,
 			break;
 
 		leaf = false;
-		ret = parse_cluster(c, package_id, i, depth + 1);
+		ret = parse_cluster(c, package_id, die_id, i, depth + 1);
 		if (depth > 0)
 			pr_warn("Topology for clusters of clusters not yet supported\n");
 		if (ret != 0)
@@ -598,7 +600,8 @@ static int __init parse_cluster(struct device_node *cluster, int package_id,
 		}
 
 		if (leaf) {
-			ret = parse_core(c, package_id, cluster_id, core_id++);
+			ret = parse_core(c, package_id, die_id, cluster_id,
+					 core_id++);
 			if (ret != 0)
 				return ret;
 		} else {
@@ -616,6 +619,34 @@ static int __init parse_cluster(struct device_node *cluster, int package_id,
 	return 0;
 }
 
+static int __init parse_die(struct device_node *parent, int package_id)
+{
+	char name[20];
+	bool has_die = false;
+	int die_id = 0, ret;
+
+	do {
+		snprintf(name, sizeof(name), "die%d", die_id);
+		struct device_node *d __free(device_node) =
+			of_get_child_by_name(parent, name);
+
+		if (!d)
+			break;
+
+		has_die = true;
+		ret = parse_cluster(d, package_id, die_id, -1, 0);
+		if (ret)
+			return ret;
+
+		die_id++;
+	} while (1);
+
+	if (!has_die)
+		ret = parse_cluster(parent, package_id, -1, -1, 0);
+
+	return ret;
+}
+
 static int __init parse_socket(struct device_node *socket)
 {
 	char name[20];
@@ -631,7 +662,7 @@ static int __init parse_socket(struct device_node *socket)
 			break;
 
 		has_socket = true;
-		ret = parse_cluster(c, package_id, -1, 0);
+		ret = parse_die(c, package_id);
 		if (ret != 0)
 			return ret;
 
@@ -639,7 +670,7 @@ static int __init parse_socket(struct device_node *socket)
 	} while (1);
 
 	if (!has_socket)
-		ret = parse_cluster(socket, 0, -1, 0);
+		ret = parse_die(socket, 0);
 
 	/*
 	 * Reset the max_smt_thread_num to 1 on failure. Since on failure
@@ -667,10 +698,7 @@ static int __init parse_dt_topology(void)
 		return 0;
 	}
 
-	/*
-	 * When topology is provided cpu-map is essentially a root
-	 * cluster with restricted subnodes.
-	 */
+	/* cpu-map is the root of the processor topology hierarchy. */
 	struct device_node *map __free(device_node) =
 		of_get_child_by_name(cn, "cpu-map");
 

-- 
2.55.0

Re: [PATCH RFC 2/3] arch_topology: Parse die nodes in /cpu-map

[Sudeep Holla]

On Sun, Sep 20, 2026 at 11:18:03AM +0800, Troy Mitchell wrote:
> The devicetree CPU map can distinguish packages and clusters but cannot
> describe multiple dies in one package. Such systems have to flatten all
> clusters into the package, making cluster identifiers package-wide and
> hiding the die boundary.
> 
> Recognize an optional sequence of dieN nodes between a socket and its
> clusters, and propagate the die identifier to every CPU below it. Enumerate
> clusters independently below each die.
> 
> When a CPU map has no dieN nodes, continue parsing its existing package
> and cluster hierarchy but pass -1 as the die identifier. Legacy maps
> therefore remain compatible without being reported as an explicitly
> described die 0.
> 

Where is the associated bindings ? Am I missing to check it in the series
or it is really missing ? It is essential to add any code using the binding.

Also how does it align with ACPI implementation ? It is preferred to be
in parity here, so we need some binding there as well IMHO.

-- 
Regards,
Sudeep

Re: [PATCH RFC 2/3] arch_topology: Parse die nodes in /cpu-map

[Troy Mitchell]

[-- Attachment #1: Type: text/plain, Size: 1306 bytes --]

> Where is the associated bindings ? Am I missing to check it in the series
> or it is really missing ? It is essential to add any code using the binding.

I left the binding out to get initial feedback on the topology model,
but I agree it should be available for review alongside the code.

The cpu-map binding lives in dt-schema. I have now opened a draft PR [1]
with the proposed dieN hierarchy. The PR also links back to this kernel
series.

> Also how does it align with ACPI implementation ? It is preferred to be
> in parity here, so we need some binding there as well IMHO.

This series does not implement ACPI die discovery and leaves die_id at
-1 on that path. I agree that DT and ACPI should describe the same
topology model.

RISC-V does not define topology fields in hart IDs. PPTT can describe
intermediate processor hierarchy nodes, but I could not find an explicit
die indicator in the ACPI 6.6 processor-node flags. Is there an existing
PPTT convention for identifying die boundaries that we should follow?

Would it be acceptable to add DT support first, keeping ACPI die IDs
unknown until that mapping is agreed, or should the ACPI mapping be
defined as part of this RFC?

Link: https://github.com/devicetree-org/dt-schema/pull/208 [1].

-- 
Troy Mitchell

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 273 bytes --]

[PATCH RFC 3/3] riscv: topology: Expose explicit CPU die topology

[Troy Mitchell]

RISC-V uses the generic CPU topology model but does not provide die
topology accessors. The generic sysfs topology code consequently cannot
expose die identifiers or die sibling masks when firmware describes them.

Simply adding the accessors would create die_id, die_cpus, and
die_cpus_list on every RISC-V CPU. That would also expose -1 and a
single-CPU mask when neither DT nor ACPI supplied a die hierarchy.

Add an architecture-selectable visibility predicate for the die sysfs
attributes. Its default preserves the existing sysfs ABI on architectures
that already expose die topology. Make RISC-V expose the attributes only
when the generic topology parser recorded a non-negative die ID.

Legacy DTs, the current ACPI path, and the fallback topology leave die_id
at -1, so RISC-V does not create die sysfs files for them.

Signed-off-by: Troy Mitchell <troy.mitchell@linux.dev>
---
 arch/riscv/include/asm/topology.h |  4 ++++
 drivers/base/topology.c           | 28 +++++++++++++++++++++++++++-
 include/linux/topology.h          |  3 +++
 3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/topology.h b/arch/riscv/include/asm/topology.h
index fe1a8bf6902d9..5d99cb39f2819 100644
--- a/arch/riscv/include/asm/topology.h
+++ b/arch/riscv/include/asm/topology.h
@@ -8,6 +8,10 @@
 #include <asm/numa.h>
 #endif
 
+#define topology_die_id(cpu)		(cpu_topology[cpu].die_id)
+#define topology_die_cpumask(cpu)	(&cpu_topology[cpu].die_sibling)
+#define topology_die_sysfs_visible(cpu)	(topology_die_id(cpu) >= 0)
+
 /* Replace task scheduler's default frequency-invariant accounting */
 #define arch_scale_freq_tick		topology_scale_freq_tick
 #define arch_set_freq_scale		topology_set_freq_scale
diff --git a/drivers/base/topology.c b/drivers/base/topology.c
index c890e2a5b4280..74d5bb979b1b8 100644
--- a/drivers/base/topology.c
+++ b/drivers/base/topology.c
@@ -171,16 +171,42 @@ static struct attribute *default_attrs[] = {
 static umode_t topology_is_visible(struct kobject *kobj,
 				   struct attribute *attr, int unused)
 {
-	if (attr == &dev_attr_ppin.attr && !topology_ppin(kobj_to_dev(kobj)->id))
+	unsigned int cpu = kobj_to_dev(kobj)->id;
+
+#ifdef TOPOLOGY_DIE_SYSFS
+	if (attr == &dev_attr_die_id.attr &&
+	    !topology_die_sysfs_visible(cpu))
+		return 0;
+#endif
+
+	if (attr == &dev_attr_ppin.attr && !topology_ppin(cpu))
 		return 0;
 
 	return attr->mode;
 }
 
+#ifdef TOPOLOGY_DIE_SYSFS
+static umode_t topology_bin_is_visible(struct kobject *kobj,
+				       const struct bin_attribute *attr, int unused)
+{
+	unsigned int cpu = kobj_to_dev(kobj)->id;
+
+	if ((attr == &bin_attr_die_cpus ||
+	     attr == &bin_attr_die_cpus_list) &&
+	    !topology_die_sysfs_visible(cpu))
+		return 0;
+
+	return attr->attr.mode;
+}
+#endif
+
 static const struct attribute_group topology_attr_group = {
 	.attrs = default_attrs,
 	.bin_attrs = bin_attrs,
 	.is_visible = topology_is_visible,
+#ifdef TOPOLOGY_DIE_SYSFS
+	.is_bin_visible = topology_bin_is_visible,
+#endif
 	.name = "topology"
 };
 
diff --git a/include/linux/topology.h b/include/linux/topology.h
index 709a2dcf4c738..9c2c9096aff36 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -179,6 +179,9 @@ static inline int cpu_to_mem(int cpu)
 
 #if defined(topology_die_id) && defined(topology_die_cpumask)
 #define TOPOLOGY_DIE_SYSFS
+#ifndef topology_die_sysfs_visible
+#define topology_die_sysfs_visible(cpu)	((void)(cpu), true)
+#endif
 #endif
 #if defined(topology_cluster_id) && defined(topology_cluster_cpumask)
 #define TOPOLOGY_CLUSTER_SYSFS

-- 
2.55.0