[Xen-devel] [PATCH v4 15/15] docs: add MBA description in docs

[Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>]

This patch adds MBA description in related documents.

Signed-off-by: Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>
Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
---
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>

v4:
    - modify description of MBA in 'xl.pod.1.in' to be same as feature doc.
      (suggested by Roger Pau Monné)
    - fix words issue.
      (suggested by Roger Pau Monné)
v2:
    - state the value type shown by 'psr-mba-show'. For linear mode,
      it shows decimal value. For non-linear mode, it shows hexadecimal
      value.
      (suggested by Chao Peng)
---
 docs/man/xl.pod.1.in      | 33 +++++++++++++++++++++++++
 docs/misc/xl-psr.markdown | 63 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 96 insertions(+)

diff --git a/docs/man/xl.pod.1.in b/docs/man/xl.pod.1.in
index 3d5f2f7..64d87ba 100644
--- a/docs/man/xl.pod.1.in
+++ b/docs/man/xl.pod.1.in
@@ -1803,6 +1803,39 @@ processed.
 
 =back
 
+=head2 Memory Bandwidth Allocation
+
+Intel Skylake and later server platforms offer capabilities to configure and
+make use of the Memory Bandwidth Allocation (MBA) mechanisms, which provides
+OS/VMMs the ability to slow misbehaving apps/VMs by using a credit-based
+throttling mechanism. In the Xen implementation, MBA is used to control memory
+bandwidth on VM basis. To enforce bandwidth on a specific domain, just set
+throttling value (THRTL) for the domain.
+
+=over 4
+
+=item B<psr-mba-set> [I<OPTIONS>] I<domain-id> I<thrtl>
+
+Set throttling value (THRTL) for a domain. For how to specify I<thrtl>
+please refer to L<http://xenbits.xen.org/docs/unstable/misc/xl-psr.html>.
+
+B<OPTIONS>
+
+=over 4
+
+=item B<-s SOCKET>, B<--socket=SOCKET>
+
+Specify the socket to process, otherwise all sockets are processed.
+
+=back
+
+=item B<psr-mba-show> [I<domain-id>]
+
+Show MBA settings for a certain domain or all domains. For linear mode, it
+shows the decimal value. For non-linear mode, it shows hexadecimal value.
+
+=back
+
 =head1 IGNORED FOR COMPATIBILITY WITH XM
 
 xl is mostly command-line compatible with the old xm utility used with
diff --git a/docs/misc/xl-psr.markdown b/docs/misc/xl-psr.markdown
index 04dd957..08c06dc 100644
--- a/docs/misc/xl-psr.markdown
+++ b/docs/misc/xl-psr.markdown
@@ -186,6 +186,69 @@ Setting data CBM for a domain:
 Setting the same code and data CBM for a domain:
 `xl psr-cat-set <domid> <cbm>`
 
+## Memory Bandwidth Allocation (MBA)
+
+Memory Bandwidth Allocation (MBA) is a new feature available on Intel
+Skylake and later server platforms that allows an OS or Hypervisor/VMM to
+slow misbehaving apps/VMs or create advanced closed-loop control system via
+exposing control over a credit-based throttling mechanism. To enforce bandwidth
+on a specific domain, just set throttling value (THRTL) into Class of Service
+(COS). MBA provides two THRTL mode. One is linear mode and the other is
+non-linear mode.
+
+In the linear mode the input precision is defined as 100-(THRTL_MAX). Values
+not an even multiple of the precision (e.g., 12%) will be rounded down (e.g.,
+to 10% delay by the hardware).
+
+If linear values are not supported then input delay values are powers-of-two
+from zero to the THRTL_MAX value from CPUID. In this case any values not a 
power
+of two will be rounded down the next nearest power of two.
+
+For example, assuming a system with 2 domains:
+
+ * A THRTL of 0x0 for every domain means each domain can access the whole cache
+   without any delay. This is the default.
+
+ * Linear mode: Giving one domain a THRTL of 0xC and the other domain's 0 means
+   that the first domain gets 10% delay to access the cache and the other one
+   without any delay.
+
+ * Non-linear mode: Giving one domain a THRTL of 0xC and the other domain's 0
+   means that the first domain gets 8% delay to access the cache and the other
+   one without any delay.
+
+For more detailed information please refer to Intel SDM chapter
+"Introduction to Memory Bandwidth Allocation".
+
+In Xen's implementation, THRTL can be configured with libxl/xl interfaces but
+COS is maintained in hypervisor only. The cache partition granularity is per
+domain, each domain has COS=0 assigned by default, the corresponding THRTL is
+0, which means all the cache resource can be accessed without delay.
+
+### xl interfaces
+
+System MBA information such as maximum COS and maximum THRTL can be obtained 
by:
+
+`xl psr-hwinfo --mba`
+
+The simplest way to change a domain's THRTL from its default is running:
+
+`xl psr-mba-set  [OPTIONS] <domid> <thrtl>`
+
+In a multi-socket system, the same thrtl will be set on each socket by default.
+Per socket thrtl can be specified with the `--socket SOCKET` option.
+
+Setting the THRTL may not be successful if insufficient COS is available. In
+such case unused COS(es) may be freed by setting THRTL of all related domains 
to
+its default value(0).
+
+Per domain THRTL settings can be shown by:
+
+`xl psr-mba-show [OPTIONS] <domid>`
+
+For linear mode, it shows the decimal value. For non-linear mode, it shows
+hexadecimal value.
+
 ## Reference
 
 [1] Intel SDM
-- 
1.9.1


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