[xen master] docs: create Memory Bandwidth Allocation (MBA) feature document

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[xen master] docs: create Memory Bandwidth Allocation (MBA) feature document

patchbot
commit 0062b96961c42eff0aa0ff3b00c26e484718c8fd
Author:     Yi Sun <[hidden email]>
AuthorDate: Fri Oct 20 10:50:00 2017 +0200
Commit:     Jan Beulich <[hidden email]>
CommitDate: Fri Dec 15 11:02:23 2017 +0100

    docs: create Memory Bandwidth Allocation (MBA) feature document
   
    This patch creates MBA feature document in doc/features/. It describes
    key points to implement MBA which is described in details in Intel SDM
    "Introduction to Memory Bandwidth Allocation".
   
    Signed-off-by: Yi Sun <[hidden email]>
    Reviewed-by: Roger Pau Monné <[hidden email]>
---
 docs/features/intel_psr_mba.pandoc | 297 +++++++++++++++++++++++++++++++++++++
 1 file changed, 297 insertions(+)

diff --git a/docs/features/intel_psr_mba.pandoc b/docs/features/intel_psr_mba.pandoc
new file mode 100644
index 0000000..86df661
--- /dev/null
+++ b/docs/features/intel_psr_mba.pandoc
@@ -0,0 +1,297 @@
+% 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.
+---------- -------- -------- -------------------------------------------
--
generated by git-patchbot for /home/xen/git/xen.git#master


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