[PATCH v5 3/4] KVM: stats: Add documentation for statistics data binary interface

Jing Zhang
2021-05-17T16:24:06Z

Update KVM API documentation for binary statistics.

Signed-off-by: Jing Zhang <jingzhangos@google.com>
---
Documentation/virt/kvm/api.rst | 171 +++++++++++++++++++++++++++++++++
1 file changed, 171 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 7fcb2fd38f42..9a6aa9770dfd 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -5034,6 +5034,169 @@ see KVM_XEN_VCPU_SET_ATTR above.
The KVM_XEN_VCPU_ATTR_TYPE_RUNSTATE_ADJUST type may not be used
with the KVM_XEN_VCPU_GET_ATTR ioctl.

+4.130 KVM_STATS_GETFD
+---------------------
+
+:Capability: KVM_CAP_STATS_BINARY_FD
+:Architectures: all
+:Type: vm ioctl, vcpu ioctl
+:Parameters: none
+:Returns: statistics file descriptor on success, < 0 on error
+
+Errors:
+
+ ====== ======================================================
+ ENOMEM if the fd could not be created due to lack of memory
+ EMFILE if the number of opened files exceeds the limit
+ ====== ======================================================
+
+The file descriptor can be used to read VM/vCPU statistics data in binary
+format. The file data is organized into three blocks as below:
++-------------+
+| Header |
++-------------+
+| Descriptors |
++-------------+
+| Stats Data |
++-------------+
+
+The Header block is always at the start of the file. It is only needed to be
+read one time after a system boot.
+It is in the form of ``struct kvm_stats_header`` as below::
+
+ #define KVM_STATS_ID_MAXLEN 64
+
+ struct kvm_stats_header {
+ char id[KVM_STATS_ID_MAXLEN];
+ __u32 name_size;
+ __u32 count;
+ __u32 desc_offset;
+ __u32 data_offset;
+ };
+
+The ``id`` field is identification for the corresponding KVM statistics. For
+KVM statistics, it is in the form of "kvm-{kvm pid}", like "kvm-12345". For
+VCPU statistics, it is in the form of "kvm-{kvm pid}/vcpu-{vcpu id}", like
+"kvm-12345/vcpu-12".
+
+The ``name_size`` field is the size (byte) of the statistics name string
+(including trailing '\0') appended to the end of every statistics descriptor.
+
+The ``count`` field is the number of statistics.
+
+The ``desc_offset`` field is the offset of the Descriptors block from the start
+of the file indicated by the file descriptor.
+
+The ``data_offset`` field is the offset of the Stats Data block from the start
+of the file indicated by the file descriptor.
+
+The Descriptors block is only needed to be read once after a system boot. It is
+an array of ``struct kvm_stats_desc`` as below::
+
+ #define KVM_STATS_TYPE_SHIFT 0
+ #define KVM_STATS_TYPE_MASK (0xF << KVM_STATS_TYPE_SHIFT)
+ #define KVM_STATS_TYPE_CUMULATIVE (0x0 << KVM_STATS_TYPE_SHIFT)
+ #define KVM_STATS_TYPE_INSTANT (0x1 << KVM_STATS_TYPE_SHIFT)
+ #define KVM_STATS_TYPE_MAX KVM_STATS_TYPE_INSTANT
+
+ #define KVM_STATS_UNIT_SHIFT 4
+ #define KVM_STATS_UNIT_MASK (0xF << KVM_STATS_UNIT_SHIFT)
+ #define KVM_STATS_UNIT_NONE (0x0 << KVM_STATS_UNIT_SHIFT)
+ #define KVM_STATS_UNIT_BYTES (0x1 << KVM_STATS_UNIT_SHIFT)
+ #define KVM_STATS_UNIT_SECONDS (0x2 << KVM_STATS_UNIT_SHIFT)
+ #define KVM_STATS_UNIT_CYCLES (0x3 << KVM_STATS_UNIT_SHIFT)
+ #define KVM_STATS_UNIT_MAX KVM_STATS_UNIT_CYCLES
+
+ #define KVM_STATS_SCALE_SHIFT 8
+ #define KVM_STATS_SCALE_MASK (0xF << KVM_STATS_SCALE_SHIFT)
+ #define KVM_STATS_SCALE_POW10 (0x0 << KVM_STATS_SCALE_SHIFT)
+ #define KVM_STATS_SCALE_POW2 (0x1 << KVM_STATS_SCALE_SHIFT)
+ #define KVM_STATS_SCALE_MAX KVM_STATS_SCALE_POW2
+
+ struct kvm_stats_desc {
+ __u32 flags;
+ __s16 exponent;
+ __u16 size;
+ __u32 unused1;
+ __u32 unused2;
+ char name[0];
+ };
+
+The ``flags`` field contains the type and unit of the statistics data described
+by this descriptor. The following flags are supported:
+ * ``KVM_STATS_TYPE_CUMULATIVE``
+ The statistics data is cumulative. The value of data can only be increased.
+ Most of the counters used in KVM are of this type.
+ The corresponding ``count`` filed for this type is always 1.
+ * ``KVM_STATS_TYPE_INSTANT``
+ The statistics data is instantaneous. Its value can be increased or
+ decreased. This type is usually used as a measurement of some resources,
+ like the number of dirty pages, the number of large pages, etc.
+ The corresponding ``count`` field for this type is always 1.
+ * ``KVM_STATS_UNIT_NONE``
+ There is no unit for the value of statistics data. This usually means that
+ the value is a simple counter of an event.
+ * ``KVM_STATS_UNIT_BYTES``
+ It indicates that the statistics data is used to measure memory size, in the
+ unit of Byte, KiByte, MiByte, GiByte, etc. The unit of the data is
+ determined by the ``exponent`` field in the descriptor. The
+ ``KVM_STATS_SCALE_POW2`` flag is valid in this case. The unit of the data is
+ determined by ``pow(2, exponent)``. For example, if value is 10,
+ ``exponent`` is 20, which means the unit of statistics data is MiByte, we
+ can get the statistics data in the unit of Byte by
+ ``value * pow(2, exponent) = 10 * pow(2, 20) = 10 MiByte`` which is
+ 10 * 1024 * 1024 Bytes.
+ * ``KVM_STATS_UNIT_SECONDS``
+ It indicates that the statistics data is used to measure time/latency, in
+ the unit of nanosecond, microsecond, millisecond and second. The unit of the
+ data is determined by the ``exponent`` field in the descriptor. The
+ ``KVM_STATS_SCALE_POW10`` flag is valid in this case. The unit of the data
+ is determined by ``pow(10, exponent)``. For example, if value is 2000000,
+ ``exponent`` is -6, which means the unit of statistics data is microsecond,
+ we can get the statistics data in the unit of second by
+ ``value * pow(10, exponent) = 2000000 * pow(10, -6) = 2 seconds``.
+ * ``KVM_STATS_UNIT_CYCLES``
+ It indicates that the statistics data is used to measure CPU clock cycles.
+ The ``KVM_STATS_SCALE_POW10`` flag is valid in this case. For example, if
+ value is 200, ``exponent`` is 4, we can get the number of CPU clock cycles
+ by ``value * pow(10, exponent) = 200 * pow(10, 4) = 2000000``.
+
+The ``exponent`` field is the scale of corresponding statistics data. It has two
+values as follows:
+ * ``KVM_STATS_SCALE_POW10``
+ The scale is based on power of 10. It is used for measurement of time and
+ CPU clock cycles.
+ * ``KVM_STATS_SCALE_POW2``
+ The scale is based on power of 2. It is used for measurement of memory size.
+
+The ``size`` field is the number of values of this statistics data. It is in the
+unit of ``unsigned long`` for VCPU or ``__u64`` for VM.
+
+The ``unused1`` and ``unused2`` fields are reserved for future
+support for other types of statistics data, like log/linear histogram.
+
+The ``name`` field points to the name string of the statistics data. The name
+string starts at the end of ``struct kvm_stats_desc``.
+The maximum length (including trailing '\0') is indicated by ``name_size``
+in ``struct kvm_stats_header``.
+
+The Stats Data block contains an array of data values of type ``struct
+kvm_vm_stats_data`` or ``struct kvm_vcpu_stats_data``. It would be read by
+user space periodically to pull statistics data.
+The order of data value in Stats Data block is the same as the order of
+descriptors in Descriptors block.
+ * Statistics data for VM::
+
+ struct kvm_vm_stats_data {
+ unsigned long value[0];
+ };
+
+ * Statistics data for VCPU::
+
+ struct kvm_vcpu_stats_data {
+ __u64 value[0];
+ };
+
5. The kvm_run structure
========================

@@ -6891,3 +7054,11 @@ This capability is always enabled.
This capability indicates that the KVM virtual PTP service is
supported in the host. A VMM can check whether the service is
available to the guest on migration.
+
+8.33 KVM_CAP_STATS_BINARY_FD
+----------------------------
+
+:Architectures: all
+
+This capability indicates the feature that user space can create get a file
+descriptor for every VM and VCPU to read statistics data in binary format.
--
2.31.1.751.gd2f1c929bd-goog