Documentation/x86/topology: Detail CPUID leaves used for topology enumeration

Add a new section describing the different CPUID leaves and fields used
to parse topology on x86 systems.

  [ bp: Cleanups and simplifications ontop. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/20250901170418.4314-1-kprateek.nayak@amd.com

Author: kudureranganath

Documentation/arch/x86/topology.rst

@@ -141,6 +141,197 @@ Thread-related topology information in the kernel:
 
 
 
+System topology enumeration
+===========================
+
+The topology on x86 systems can be discovered using a combination of vendor
+specific CPUID leaves which enumerate the processor topology and the cache
+hierarchy.
+
+The CPUID leaves in their preferred order of parsing for each x86 vendor is as
+follows:
+
+1) AMD
+
+   1) CPUID leaf 0x80000026 [Extended CPU Topology] (Core::X86::Cpuid::ExCpuTopology)
+
+      The extended CPUID leaf 0x80000026 is the extension of the CPUID leaf 0xB
+      and provides the topology information of Core, Complex, CCD (Die), and
+      Socket in each level.
+
+      Support for the leaf is discovered by checking if the maximum extended
+      CPUID level is >= 0x80000026 and then checking if `LogProcAtThisLevel`
+      in `EBX[15:0]` at a particular level (starting from 0) is non-zero.
+
+      The `LevelType` in `ECX[15:8]` at the level provides the topology domain
+      the level describes - Core, Complex, CCD(Die), or the Socket.
+
+      The kernel uses the `CoreMaskWidth` from `EAX[4:0]` to discover the
+      number of bits that need to be right-shifted from `ExtendedLocalApicId`
+      in `EDX[31:0]` in order to get a unique Topology ID for the topology 
+      level. CPUs with the same Topology ID share the resources at that level.
+
+      CPUID leaf 0x80000026 also provides more information regarding the power
+      and efficiency rankings, and about the core type on AMD processors with
+      heterogeneous characteristics.
+
+      If CPUID leaf 0x80000026 is supported, further parsing is not required.
+
+   2) CPUID leaf 0x0000000B [Extended Topology Enumeration] (Core::X86::Cpuid::ExtTopEnum)
+
+      The extended CPUID leaf 0x0000000B is the predecessor on the extended
+      CPUID leaf 0x80000026 and only describes the core, and the socket domains
+      of the processor topology.
+
+      The support for the leaf is discovered by checking if the maximum supported
+      CPUID level is >= 0xB and then if `EBX[31:0]` at a particular level
+      (starting from 0) is non-zero.
+
+      The `LevelType` in `ECX[15:8]` at the level provides the topology domain
+      that the level describes - Thread, or Processor (Socket).
+
+      The kernel uses the `CoreMaskWidth` from `EAX[4:0]` to discover the
+      number of bits that need to be right-shifted from the `ExtendedLocalApicId`
+      in `EDX[31:0]` to get a unique Topology ID for that topology level. CPUs
+      sharing the Topology ID share the resources at that level.
+
+      If CPUID leaf 0xB is supported, further parsing is not required.
+
+
+   3) CPUID leaf 0x80000008 ECX [Size Identifiers] (Core::X86::Cpuid::SizeId)
+
+      If neither the CPUID leaf 0x80000026 nor 0xB is supported, the number of
+      CPUs on the package is detected using the Size Identifier leaf
+      0x80000008 ECX.
+
+      The support for the leaf is discovered by checking if the supported
+      extended CPUID level is >= 0x80000008.
+
+      The shifts from the APIC ID for the Socket ID is calculated from the
+      `ApicIdSize` field in `ECX[15:12]` if it is non-zero.
+
+      If `ApicIdSize` is reported to be zero, the shift is calculated as the
+      order of the `number of threads` calculated from `NC` field in
+      `ECX[7:0]` which describes the `number of threads - 1` on the package.
+
+      Unless Extended APIC ID is supported, the APIC ID used to find the
+      Socket ID is from the `LocalApicId` field of CPUID leaf 0x00000001
+      `EBX[31:24]`.
+
+      The topology parsing continues to detect if Extended APIC ID is
+      supported or not.
+
+
+   4) CPUID leaf 0x8000001E [Extended APIC ID, Core Identifiers, Node Identifiers]
+      (Core::X86::Cpuid::{ExtApicId,CoreId,NodeId})
+
+      The support for Extended APIC ID can be detected by checking for the
+      presence of `TopologyExtensions` in `ECX[22]` of CPUID leaf 0x80000001
+      [Feature Identifiers] (Core::X86::Cpuid::FeatureExtIdEcx).
+
+      If Topology Extensions is supported, the APIC ID from `ExtendedApicId`
+      from CPUID leaf 0x8000001E `EAX[31:0]` should be preferred over that from
+      `LocalApicId` field of CPUID leaf 0x00000001 `EBX[31:24]` for topology
+      enumeration.
+
+      On processors of Family 0x17 and above that do not support CPUID leaf
+      0x80000026 or CPUID leaf 0xB, the shifts from the APIC ID for the Core
+      ID is calculated using the order of `number of threads per core`
+      calculated using the `ThreadsPerCore` field in `EBX[15:8]` which
+      describes `number of threads per core - 1`.
+
+      On Processors of Family 0x15, the Core ID from `EBX[7:0]` is used as the
+      `cu_id` (Compute Unit ID) to detect CPUs that share the compute units.
+
+
+   All AMD processors that support the `TopologyExtensions` feature store the
+   `NodeId` from the `ECX[7:0]` of CPUID leaf 0x8000001E 
+   (Core::X86::Cpuid::NodeId) as the per-CPU `node_id`. On older processors,
+   the `node_id` was discovered using MSR_FAM10H_NODE_ID MSR (MSR
+   0x0xc001_100c). The presence of the NODE_ID MSR was detected by checking
+   `ECX[19]` of CPUID leaf 0x80000001 [Feature Identifiers]
+   (Core::X86::Cpuid::FeatureExtIdEcx).
+
+
+2) Intel
+
+   On Intel platforms, the CPUID leaves that enumerate the processor
+   topology are as follows:
+
+   1) CPUID leaf 0x1F (V2 Extended Topology Enumeration Leaf)
+
+      The CPUID leaf 0x1F is the extension of the CPUID leaf 0xB and provides
+      the topology information of Core, Module, Tile, Die, DieGrp, and Socket
+      in each level.
+
+      The support for the leaf is discovered by checking if the supported
+      CPUID level is >= 0x1F and then `EBX[31:0]` at a particular level
+      (starting from 0) is non-zero.
+
+      The `Domain Type` in `ECX[15:8]` of the sub-leaf provides the topology
+      domain that the level describes - Core, Module, Tile, Die, DieGrp, and
+      Socket.
+
+      The kernel uses the value from `EAX[4:0]` to discover the number of
+      bits that need to be right shifted from the `x2APIC ID` in `EDX[31:0]`
+      to get a unique Topology ID for the topology level. CPUs with the same
+      Topology ID share the resources at that level.
+
+      If CPUID leaf 0x1F is supported, further parsing is not required.
+
+
+   2) CPUID leaf 0x0000000B (Extended Topology Enumeration Leaf)
+
+      The extended CPUID leaf 0x0000000B is the predecessor of the V2 Extended
+      Topology Enumeration Leaf 0x1F and only describes the core, and the
+      socket domains of the processor topology.
+
+      The support for the leaf is iscovered by checking if the supported CPUID
+      level is >= 0xB and then checking if `EBX[31:0]` at a particular level
+      (starting from 0) is non-zero.
+
+      CPUID leaf 0x0000000B shares the same layout as CPUID leaf 0x1F and
+      should be enumerated in a similar manner.
+
+      If CPUID leaf 0xB is supported, further parsing is not required.
+
+
+   3) CPUID leaf 0x00000004 (Deterministic Cache Parameters Leaf)
+
+      On Intel processors that support neither CPUID leaf 0x1F, nor CPUID leaf
+      0xB, the shifts for the SMT domains is calculated using the number of
+      CPUs sharing the L1 cache.
+
+      Processors that feature Hyper-Threading is detected using `EDX[28]` of
+      CPUID leaf 0x1 (Basic CPUID Information).
+
+      The order of `Maximum number of addressable IDs for logical processors
+      sharing this cache` from `EAX[25:14]` of level-0 of CPUID 0x4 provides
+      the shifts from the APIC ID required to compute the Core ID.
+
+      The APIC ID and Package information is computed using the data from
+      CPUID leaf 0x1.
+
+
+   4) CPUID leaf 0x00000001 (Basic CPUID Information)
+
+      The mask and shifts to derive the Physical Package (socket) ID is
+      computed using the `Maximum number of addressable IDs for logical
+      processors in this physical package` from `EBX[23:16]` of CPUID leaf
+      0x1.
+
+     The APIC ID on the legacy platforms is derived from the `Initial APIC
+     ID` field from `EBX[31:24]` of CPUID leaf 0x1.
+
+
+3) Centaur and Zhaoxin
+
+   Similar to Intel, Centaur and Zhaoxin use a combination of CPUID leaf
+   0x00000004 (Deterministic Cache Parameters Leaf) and CPUID leaf 0x00000001
+   (Basic CPUID Information) to derive the topology information.
+
+
+
 System topology examples
 ========================