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+% Intel Memory Bandwidth Allocation (MBA) Feature
+% Revision 1.8
+
+\clearpage
+
+# Basics
+
+---------------- ----------------------------------------------------
+ Status: **Tech Preview**
+
+Architecture(s): Intel x86
+
+ Component(s): Hypervisor, toolstack
+
+ Hardware: MBA is supported on Skylake Server and beyond
+---------------- ----------------------------------------------------
+
+# Terminology
+
+* CAT Cache Allocation Technology
+* CBM Capacity BitMasks
+* CDP Code and Data Prioritization
+* COS/CLOS Class of Service
+* HW Hardware
+* MBA Memory Bandwidth Allocation
+* MSRs Machine Specific Registers
+* PSR Intel Platform Shared Resource
+* THRTL Throttle value or delay value
+
+# Overview
+
+The Memory Bandwidth Allocation (MBA) feature provides indirect and approximate
+control over memory bandwidth available per-core. This feature provides OS/
+hypervisor the ability to slow misbehaving apps/domains by using a credit-based
+throttling mechanism.
+
+# User details
+
+* Feature Enabling:
+
+ Add "psr=mba" to boot line parameter to enable MBA feature.
+
+* xl interfaces:
+
+ 1. `psr-mba-show [domain-id|domain-name]`:
+
+ Show memory bandwidth throttling for domain. Under different modes, it
+ shows different type of data.
+
+ There are two modes:
+ Linear mode: the input precision is defined as 100-(MBA_MAX). For instance,
+ if the MBA_MAX value is 90, the input precision is 10%. Values not an even
+ multiple of the precision (e.g., 12%) will be rounded down (e.g., to 10%
+ delay applied) by HW automatically. The response of throttling value is
+ linear.
+
+ Non-linear mode: input delay values are powers-of-two from zero to the
+ MBA_MAX value from CPUID. In this case any values not a power of two will
+ be rounded down the next nearest power of two by HW automatically. The
+ response of throttling value is non-linear.
+
+ For linear mode, it shows the decimal value. For non-linear mode, it shows
+ hexadecimal value.
+
+ 2. `psr-mba-set [OPTIONS] <domain-id|domain-name> <throttling>`:
+
+ Set memory bandwidth throttling for domain.
+
+ Options:
+ '-s': Specify the socket to process, otherwise all sockets are processed.
+
+ Throttling value set in register implies the approximate amount of delaying
+ the traffic between core and memory. Higher throttling value result in
+ lower bandwidth. The max throttling value (MBA_MAX) supported can be
+ obtained through CPUID inside hypervisor. Users can fetch the MBA_MAX value
+ using the `psr-hwinfo` xl command.
+
+# Technical details
+
+MBA is a member of Intel PSR features, it shares the base PSR infrastructure
+in Xen.
+
+## Hardware perspective
+
+ MBA defines a range of MSRs to support specifying a delay value (Thrtl) per
+ COS, with details below.
+
+ ```
+ +----------------------------+----------------+
+ | MSR (per socket) | Address |
+ +----------------------------+----------------+
+ | IA32_L2_QOS_Ext_BW_Thrtl_0 | 0xD50 |
+ +----------------------------+----------------+
+ | ... | ... |
+ +----------------------------+----------------+
+ | IA32_L2_QOS_Ext_BW_Thrtl_n | 0xD50+n |
+ +----------------------------+----------------+
+ ```
+
+ When context switch happens, the COS ID of domain is written to per-hyper-
+ thread MSR `IA32_PQR_ASSOC`, and then hardware enforces bandwidth allocation
+ according to the throttling value stored in the Thrtl MSR register.
+
+## The relationship between MBA and CAT/CDP
+
+ Generally speaking, MBA is completely independent of CAT/CDP, and any
+ combination may be applied at any time, e.g. enabling MBA with CAT
+ disabled.
+
+ But it needs to be noticed that MBA shares COS infrastructure with CAT,
+ although MBA is enumerated by different CPUID leaf from CAT (which
+ indicates that the max COS of MBA may be different from CAT). In some
+ cases, a domain 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 MBA is 8 but the max COS of L3 CAT is 16, when a domain is assigned
+ 9 as COS, the L3 CAT CBM associated to COS 9 would be enforced, but for MBA,
+ the HW works as default value is set since COS 9 is beyond the max COS (8)
+ of MBA.
+
+## Design Overview
+
+* Core COS/Thrtl association
+
+ When enforcing Memory Bandwidth Allocation, all cores of domains have
+ the same default Thrtl MSR (COS0) which stores the same Thrtl (0). The
+ default Thrtl MSR is used only in hypervisor and is transparent to tool stack
+ and user.
+
+ System administrators can change PSR allocation policy at runtime by
+ using the tool stack. Since MBA shares COS ID with CAT/CDP, a COS ID
+ corresponds to a 2-tuple, like [CBM, Thrtl] with only-CAT enabled, when CDP
+ is enabled, the COS ID corresponds to a 3-tuple, like [Code_CBM, Data_CBM,
+ Thrtl]. If neither CAT nor CDP is enabled, things are easier, since one COS
+ ID corresponds to one Thrtl.
+
+* VCPU schedule
+
+ This part reuses CAT COS infrastructure.
+
+* Multi-sockets
+
+ Different sockets may have different MBA capabilities (like max COS)
+ although it is consistent on the same socket. So the capability
+ of per-socket MBA is specified.
+
+ This part reuses CAT COS infrastructure.
+
+## Implementation Description
+
+* Hypervisor interfaces:
+
+ 1. Boot line param: "psr=mba" to enable the feature.
+
+ 2. SYSCTL:
+ - XEN_SYSCTL_PSR_MBA_get_info: Get system MBA information.
+
+ 3. DOMCTL:
+ - XEN_DOMCTL_PSR_MBA_OP_GET_THRTL: Get throttling for a domain.
+ - XEN_DOMCTL_PSR_MBA_OP_SET_THRTL: Set throttling for a domain.
+
+* xl interfaces:
+
+ 1. psr-mba-show [domain-id]
+ Show system/domain runtime MBA throttling value. For linear mode,
+ it shows the decimal value. For non-linear mode, it shows hexadecimal
+ value.
+ => XEN_SYSCTL_PSR_MBA_get_info/XEN_DOMCTL_PSR_MBA_OP_GET_THRTL
+
+ 2. psr-mba-set [OPTIONS] <domain-id> <throttling>
+ Set bandwidth throttling for a domain.
+ => XEN_DOMCTL_PSR_MBA_OP_SET_THRTL
+
+ 3. psr-hwinfo
+ Show PSR HW information, including L3 CAT/CDP/L2 CAT/MBA.
+ => XEN_SYSCTL_PSR_MBA_get_info
+
+* Key data structure:
+
+ 1. Feature HW info
+
+ ```
+ struct {
+ unsigned int thrtl_max;
+ bool linear;
+ } mba;
+
+ - Member `thrtl_max`
+
+ `thrtl_max` is the max throttling value to be set, i.e. MBA_MAX.
+
+ - Member `linear`
+
+ `linear` means the response of delay value is linear or not.
+
+ As mentioned above, MBA is a member of Intel PSR features, it shares the
+ base PSR infrastructure in Xen. For example, the 'cos_max' is a common HW
+ property for all features. So, for other data structure details, please
+ refer to 'intel_psr_cat_cdp.pandoc'.
+
+# Limitations
+
+MBA can only work on HW which supports it (check CPUID).
+
+# Testing
+
+We can execute these commands to verify MBA on different HWs supporting them.
+
+For example:
+ 1. User can get the MBA hardware info through 'psr-hwinfo' command. From
+ result, user can know if this hardware works under linear mode or non-
+ linear mode, the max throttling value (MBA_MAX) and so on.
+
+ root@:~$ xl psr-hwinfo --mba
+ Memory Bandwidth Allocation (MBA):
+ Socket ID : 0
+ Linear Mode : Enabled
+ Maximum COS : 7
+ Maximum Throttling Value: 90
+ Default Throttling Value: 0
+
+ 2. Then, user can set a throttling value to a domain. For example, set '10',
+ i.e 10% delay.
+
+ root@:~$ xl psr-mba-set 1 10
+
+ 3. User can check the current configuration of the domain through
+ 'psr-mab-show'. For linear mode, the decimal value is shown.
+
+ root@:~$ xl psr-mba-show 1
+ Socket ID : 0
+ Default THRTL : 0
+ ID NAME THRTL
+ 1 ubuntu14 10
+
+# 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
+---------- -------- -------- -------------------------------------------
+2017-01-10 1.0 Xen 4.9 Design document written
+2017-07-10 1.1 Xen 4.10 Changes:
+ 1. Modify data structure according to latest
+ codes;
+ 2. Add content for 'Areas for improvement';
+ 3. Other minor changes.
+2017-08-09 1.2 Xen 4.10 Changes:
+ 1. Remove a special character to avoid error when
+ building pandoc.
+2017-08-15 1.3 Xen 4.10 Changes:
+ 1. Add terminology 'HW'.
+ 2. Change 'COS ID of VCPU' to 'COS ID of domain'.
+ 3. Change 'COS register' to 'Thrtl MSR'.
+ 4. Explain the value shown for 'psr-mba-show' under
+ different modes.
+ 5. Remove content in 'Areas for improvement'.
+2017-08-16 1.4 Xen 4.10 Changes:
+ 1. Add '<>' for mandatory argument.
+2017-08-30 1.5 Xen 4.10 Changes:
+ 1. Modify words in 'Overview' to make it easier to
+ understand.
+ 2. Explain 'linear/non-linear' modes before mention
+ them.
+ 3. Explain throttling value more accurate.
+ 4. Explain 'MBA_MAX'.
+ 5. Correct some words in 'Design Overview'.
+ 6. Change 'mba_info' to 'mba' according to code
+ changes. Also, modify contents of it.
+ 7. Add context in 'Testing' part to make things
+ more clear.
+ 8. Remove 'n<64' to avoid out-of-sync.
+2017-09-21 1.6 Xen 4.10 Changes:
+ 1. Add 'domain-name' as parameter of 'psr-mba-show/
+ psr-mba-set'.
+ 2. Fix some wordings.
+ 3. Explain how user can know the MBA_MAX.
+ 4. Move the description of 'Linear mode/Non-linear
+ mode' into section of 'psr-mba-show'.
+ 5. Change 'per-thread' to 'per-hyper-thread'.
+2017-09-29 1.7 Xen 4.10 Changes:
+ 1. Correct some words.
+ 2. Change 'xl psr-mba-set 1 0xa' to
+ 'xl psr-mba-set 1 10'
+2017-10-08 1.8 Xen 4.10 Changes:
+ 1. Correct some words.
+---------- -------- -------- -------------------------------------------