[PATCH v8 3/4] KVM: stats: Add documentation for statistics data binary interface

[Jing Zhang <jingzhangos@google.com>]

Update KVM API documentation for binary statistics.

Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Signed-off-by: Jing Zhang <jingzhangos@google.com>
---
 Documentation/virt/kvm/api.rst | 174 ++++++++++++++++++++++++++++++++-
 1 file changed, 173 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index cded99561adf..4ccad2e449e2 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -5034,7 +5034,6 @@ 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.
 
-
 4.131 KVM_GET_SREGS2
 ------------------
 
@@ -5081,6 +5080,171 @@ Writes special registers into the vcpu.
 See KVM_GET_SREGS2 for the data structures.
 This ioctl (when supported) replaces the KVM_SET_SREGS.
 
+4.133 KVM_GET_STATS_FD
+---------------------
+
+: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 for the lifetime of the file descriptor.
+It is in the form of ``struct kvm_stats_header`` as below::
+
+	#define KVM_STATS_ID_MAXLEN		64
+
+	struct kvm_stats_header {
+		__u32 name_size;
+		__u32 count;
+		__u32 desc_offset;
+		__u32 data_offset;
+		char id[0];
+	};
+
+The ``id`` field is identification for the corresponding KVM statistics. For
+VM 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 for the lifetime of the
+file descriptor. It is an array of ``struct kvm_stats_desc`` as shown in
+below code block::
+
+	#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_BASE_SHIFT		8
+	#define KVM_STATS_BASE_MASK		(0xF << KVM_STATS_BASE_SHIFT)
+	#define KVM_STATS_BASE_POW10		(0x0 << KVM_STATS_BASE_SHIFT)
+	#define KVM_STATS_BASE_POW2		(0x1 << KVM_STATS_BASE_SHIFT)
+	#define KVM_STATS_BASE_MAX		KVM_STATS_BASE_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:
+
+Bits 0-3 of ``flags`` encode the type:
+  * ``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.
+
+Bits 4-7 of ``flags`` encode the unit:
+  * ``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_BASE_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_BASE_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_BASE_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``.
+
+Bits 7-11 of ``flags`` encode the base:
+  * ``KVM_STATS_BASE_POW10``
+    The scale is based on power of 10. It is used for measurement of time and
+    CPU clock cycles.
+  * ``KVM_STATS_BASE_POW2``
+    The scale is based on power of 2. It is used for measurement of memory size.
+
+The ``exponent`` field is the scale of corresponding statistics data. For
+example, if the unit is ``KVM_STATS_UNIT_BYTES``, the base is
+``KVM_STATS_BASE_POW2``, the ``exponent`` is 10, then we know that the real
+unit of the statistics data is KBytes a.k.a pow(2, 10) = 1024 bytes.
+
+The ``size`` field is the number of values of this statistics data. It is in the
+unit of ``unsigned long`` for VM or ``__u64`` for VCPU.
+
+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/VCPU::
+
+	struct kvm_stats_data {
+		__u64 value[0];
+	};
 
 5. The kvm_run structure
 ========================
@@ -6950,3 +7114,11 @@ guest according to the bits Hyper-V CPUID feature leaves. Otherwise, all
 currently implmented Hyper-V features are provided unconditionally when
 Hyper-V identification is set in the HYPERV_CPUID_INTERFACE (0x40000001)
 leaf.
+
+8.34 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.
-- 
2.32.0.272.g935e593368-goog