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+% Intel L2 Cache Allocation Technology (L2 CAT) Feature
+% Revision 1.0
+
+\clearpage
+
+# Basics
+
+---------------- ----------------------------------------------------
+         Status: **Tech Preview**
+
+Architecture(s): Intel x86
+
+   Component(s): Hypervisor, toolstack
+
+       Hardware: Atom codename Goldmont and beyond CPUs
+---------------- ----------------------------------------------------
+
+# Overview
+
+L2 CAT allows an OS or Hypervisor/VMM to control allocation of a
+CPU's shared L2 cache based on application priority or Class of Service
+(COS). Each CLOS is configured using capacity bitmasks (CBM) which
+represent cache capacity and indicate the degree of overlap and
+isolation between classes. Once L2 CAT is configured, the processor
+allows access to portions of L2 cache according to the established
+class of service.
+
+## Terminology
+
+* CAT         Cache Allocation Technology
+* CBM         Capacity BitMasks
+* CDP         Code and Data Prioritization
+* COS/CLOS    Class of Service
+* MSRs        Machine Specific Registers
+* PSR         Intel Platform Shared Resource
+* VMM         Virtual Machine Monitor
+
+# User details
+
+* Feature Enabling:
+
+  Add "psr=cat" to boot line parameter to enable all supported level CAT
+  features.
+
+* xl interfaces:
+
+  1. `psr-cat-show [OPTIONS] domain-id`:
+
+     Show domain L2 or L3 CAT CBM.
+
+     New option `-l` is added.
+     `-l2`: Show cbm for L2 cache.
+     `-l3`: Show cbm for L3 cache.
+
+     If neither `-l2` nor `-l3` is given, show both of them. If any one
+     is not supported, will print error info.
+
+  2. `psr-cat-cbm-set [OPTIONS] domain-id cbm`:
+
+     Set domain L2 or L3 CBM.
+
+     New option `-l` is added.
+     `-l2`: Specify cbm for L2 cache.
+     `-l3`: Specify cbm for L3 cache.
+
+     If neither `-l2` nor `-l3` is given, level 3 is the default option.
+
+  3. `psr-hwinfo [OPTIONS]`:
+
+     Show L2 & L3 CAT HW informations on every socket.
+
+# Technical details
+
+L2 CAT is a member of Intel PSR features and part of CAT, it shares
+some base PSR infrastructure in Xen.
+
+## Hardware perspective
+
+L2 CAT defines a new range MSRs to assign different L2 cache access
+patterns which are known as CBMs, each CBM is associated with a COS.
+
+```
+
+                        +----------------------------+----------------+
+   IA32_PQR_ASSOC       | MSR (per socket)           |    Address     |
+ +----+---+-------+     +----------------------------+----------------+
+ |    |COS|       |     | IA32_L2_QOS_MASK_0         |     0xD10      |
+ +----+---+-------+     +----------------------------+----------------+
+        └-------------> | ...                        |  ...           |
+                        +----------------------------+----------------+
+                        | IA32_L2_QOS_MASK_n         | 0xD10+n (n<64) |
+                        +----------------------------+----------------+
+```
+
+When context switch happens, the COS of VCPU is written to per-thread
+MSR `IA32_PQR_ASSOC`, and then hardware enforces L2 cache allocation
+according to the corresponding CBM.
+
+## The relationship between L2 CAT and L3 CAT/CDP
+
+L2 CAT is independent of L3 CAT/CDP, which means L2 CAT would be enabled
+while L3 CAT/CDP is disabled, or L2 CAT and L3 CAT/CDP are all enabled.
+
+L2 CAT uses a new range CBMs from 0xD10 ~ 0xD10+n (n<64), following by
+the L3 CAT/CDP CBMs, and supports setting different L2 cache accessing
+patterns from L3 cache. Like L3 CAT/CDP requirement, the bits of CBM of
+L2 CAT must be continuous too.
+
+N.B. L2 CAT and L3 CAT/CDP share the same COS field in the same
+associate register `IA32_PQR_ASSOC`, which means one COS associates to a
+pair of L2 CBM and L3 CBM.
+
+Besides, the max COS of L2 CAT may be different from L3 CAT/CDP (or
+other PSR features in future). In some cases, a VM is permitted to have a
+COS that is beyond one (or more) of PSR features but within the others.
+For instance, let's assume the max COS of L2 CAT is 8 but the max COS of
+L3 CAT is 16, when a VM is assigned 9 as COS, the L3 CBM associated to
+COS 9 would be enforced, but for L2 CAT, the behavior is fully open (no
+limit) since COS 9 is beyond the max COS (8) of L2 CAT.
+
+## Design Overview
+
+* Core COS/CBM association
+
+  When enforcing L2 CAT, all cores of domains have the same default
+  COS (COS0) which associated to the fully open CBM (all ones bitmask)
+  to access all L2 cache. The default COS is used only in hypervisor
+  and is transparent to tool stack and user.
+
+  System administrator can change PSR allocation policy at runtime by
+  tool stack. Since L2 CAT share COS with L3 CAT/CDP, a COS corresponds
+  to a 2-tuple, like [L2 CBM, L3 CBM] with only-CAT enabled, when CDP
+  is enabled, one COS corresponds to a 3-tuple, like [L2 CBM,
+  L3 Code_CBM, L3 Data_CBM]. If neither L3 CAT nor L3 CDP is enabled,
+  things would be easier, one COS corresponds to one L2 CBM.
+
+* VCPU schedule
+
+  This part reuses L3 CAT COS infrastructure.
+
+* Multi-sockets
+
+  Different sockets may have different L2 CAT capability (e.g. max COS)
+  although it is consistent on the same socket. So the capability of
+  per-socket L2 CAT is specified.
+
+## Implementation Description
+
+* Hypervisor interfaces:
+
+  1. Boot line parameter "psr=cat" now will enable L2 CAT and L3
+     CAT if hardware supported.
+
+  2. SYSCTL:
+          - XEN_SYSCTL_PSR_CAT_get_l2_info: Get L2 CAT information.
+
+  3. DOMCTL:
+          - XEN_DOMCTL_PSR_CAT_OP_GET_L2_CBM: Get L2 CBM for a domain.
+          - XEN_DOMCTL_PSR_CAT_OP_SET_L2_CBM: Set L2 CBM for a domain.
+
+* xl interfaces:
+
+  1. psr-cat-show -l2 domain-id
+          Show L2 cbm for a domain.
+          => XEN_SYSCTL_PSR_CAT_get_l2_info /
+             XEN_DOMCTL_PSR_CAT_OP_GET_L2_CBM
+
+  2. psr-mba-set -l2 domain-id cbm
+          Set L2 cbm for a domain.
+          => XEN_DOMCTL_PSR_CAT_OP_SET_L2_CBM
+
+  3. psr-hwinfo
+          Show PSR HW information, including L2 CAT
+          => XEN_SYSCTL_PSR_CAT_get_l2_info
+
+* Key data structure:
+
+   1. Feature HW info
+
+      ```
+      struct psr_cat_hw_info {
+          unsigned int cbm_len;
+          unsigned int cos_max;
+      };
+      ```
+
+      - Member `cbm_len`
+
+        `cbm_len` is one of the hardware info of CAT.
+
+      - Member `cos_max`
+
+        `cos_max` is one of the hardware info of CAT.
+
+   2. Feature list node
+
+      ```
+      struct feat_node {
+          enum psr_feat_type feature;
+          struct feat_ops ops;
+          struct psr_cat_hw_info info;
+          uint64_t cos_reg_val[MAX_COS_REG_NUM];
+          struct list_head list;
+      };
+      ```
+
+      When a PSR enforcement feature is enabled, it will be added into a
+      feature list. The head of the list is created in psr initialization.
+
+      - Member `feature`
+
+        `feature` is an integer number, to indicate which feature the list entry
+        corresponds to.
+
+      - Member `ops`
+
+        `ops` maintains a callback function list of the feature. It will be introduced
+        in details later.
+
+      - Member `info`
+
+        `info` maintains the feature HW information which can be got through
+        psr_hwinfo command.
+
+      - Member `cos_reg_val`
+
+        `cos_reg_val` is an array to maintain the value set in all COS registers of
+        the feature.
+
+   3. Per-socket PSR features information structure
+
+      ```
+      struct psr_cat_socket_info {
+          unsigned int feat_mask;
+          unsigned int nr_feat;
+          struct list_head feat_list;
+          unsigned int cos_ref[MAX_COS_REG_NUM];
+          spinlock_t ref_lock;
+      };
+      ```
+
+      We collect all PSR allocation features information of a socket in
+      this `struct psr_cat_socket_info`.
+
+      - Member `feat_mask`
+
+        `feat_mask` is a bitmap, to indicate which feature is enabled on
+        current socket. We define `feat_mask` bitmap as:
+
+        bit 0~1: L3 CAT status, [01] stands for L3 CAT only and [10]
+                 stands for L3 CDP is enalbed.
+
+        bit 2: L2 CAT status.
+
+      - Member `cos_ref`
+
+        `cos_ref` is an array which maintains the reference of one COS.
+        If the COS is used by one domain, the reference will increase one.
+        If a domain releases the COS, the reference will decrease one. The
+        array is indexed by COS.
+
+   4. Feature operation functions structure
+
+      ```
+      struct feat_ops {
+          void (*init_feature)(unsigned int eax, unsigned int ebx,
+                               unsigned int ecx, unsigned int edx,
+                               struct feat_node *feat,
+                               struct psr_cat_socket_info *info);
+          int (*get_feat_info)(const struct feat_node *feat, enum cbm_type type,
+                               uint32_t dat[], uint32_t array_len);
+          int (*get_val)(const struct feat_node *feat, unsigned int cos,
+                         enum cbm_type type, uint64_t *val);
+          unsigned int (*get_max_cos_max)(const struct feat_node *feat);
+          unsigned int (*get_cos_num)(const struct feat_node *feat);
+          int (*get_old_val)(uint64_t val[],
+                             const struct feat_node *feat,
+                             unsigned int old_cos);
+          int (*set_new_val)(uint64_t val[],
+                             const struct feat_node *feat,
+                             unsigned int old_cos,
+                             enum cbm_type type,
+                             uint64_t m);
+          int (*compare_val)(const uint64_t val[], const struct feat_node *feat,
+                             unsigned int cos, bool *found);
+          unsigned int (*get_cos_max_from_type)(const struct feat_node *feat,
+                                                enum cbm_type type);
+          unsigned int (*exceeds_cos_max)(const uint64_t val[],
+                                          const struct feat_node *feat,
+                                          unsigned int cos);
+          int (*write_msr)(unsigned int cos, const uint64_t val[],
+                           struct feat_node *feat);
+      };
+      ```
+
+      We abstract above callback functions to encapsulate the feature specific
+      behaviors into them. Then, it is easy to add a new feature. We just need:
+          1) Implement such ops and callback functions for every feature.
+          2) Register the ops into `struct feat_node`.
+          3) Add the feature into feature list during CPU initialization.
+
+# Limitations
+
+L2 CAT can only work on HW which enables it(check by CPUID). So far, there
+is no HW enables both L2 CAT and L3 CAT/CDP. But SW implementation has considered
+such scenario to enable both L2 CAT and L3 CAT/CDP.
+
+# Testing
+
+L2 CAT uses same xl interfaces as L3 CAT/CDP. So, we can execute these
+commands to verify L2 CAT and L3 CAT/CDP on different HWs support them.
+
+For example:
+    root@:~$ xl psr-hwinfo --cat
+    Cache Allocation Technology (CAT): L2
+    Socket ID       : 0
+    Maximum COS     : 3
+    CBM length      : 8
+    Default CBM     : 0xff
+
+    root@:~$ xl psr-cat-cbm-set -l2 1 0x7f
+
+    root@:~$ xl psr-cat-show -l2 1
+    Socket ID       : 0
+    Default CBM     : 0xff
+       ID                     NAME             CBM
+        1                 ubuntu14            0x7f
+
+# Areas for improvement
+
+N/A
+
+# Known issues
+
+N/A
+
+# References
+
+"INTEL® RESOURCE DIRECTOR TECHNOLOGY (INTEL® RDT) ALLOCATION FEATURES" [Intel® 64 and IA-32 Architectures Software Developer Manuals, vol3](http://www.intel.com/content/www/us/en/processors/architectures-software-developer-manuals.html)
+
+# History
+
+------------------------------------------------------------------------
+Date       Revision Version  Notes
+---------- -------- -------- -------------------------------------------
+2016-08-12 1.0      Xen 4.9  Design document written
+---------- -------- -------- -------------------------------------------
