This new API provides a file descriptor for every VM and VCPU to read
KVM statistics data in binary format.
It is meant to provide a lightweight, flexible, scalable and efficient
lock-free solution for user space telemetry applications to pull the
statistics data periodically for large scale systems. The pulling
frequency could be as high as a few times per second.
The statistics descriptors are defined by KVM in kernel and can be
by userspace to discover VM/VCPU statistics during the one-time setup
stage.
The statistics data itself could be read out by userspace telemetry
periodically without any extra parsing or setup effort.
There are a few existed interface protocols and definitions, but no
one can fulfil all the requirements this interface implemented as
below:
1. During high frequency periodic stats reading, there should be no
   extra efforts except the stats data read itself.
2. Support stats annotation, like type (cumulative, instantaneous,
   peak, histogram, etc) and unit (counter, time, size, cycles, etc).
3. The stats data reading should be free of lock/synchronization. We
   don't care about the consistency between all the stats data. All
   stats data can not be read out at exactly the same time. We really
   care about the change or trend of the stats data. The lock-free
   solution is not just for efficiency and scalability, also for the
   stats data accuracy and usability. For example, in the situation
   that all the stats data readings are protected by a global lock,
   if one VCPU died somehow with that lock held, then all stats data
   reading would be blocked, then we have no way from stats data that
   which VCPU has died.
4. The stats data reading workload can be handed over to other
   unprivileged process.

Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Ricardo Koller <redacted>
Reviewed-by: Krish Sadhukhan <redacted>
Reviewed-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Jing Zhang <redacted>
---
 Documentation/virt/kvm/api.rst | 176 ++++++++++++++++++++++++++++++++-
 1 file changed, 175 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index e328caa35d6c..7ca1c8d190c0 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 +5080,173 @@ 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[];
+	};
+
+The ``id`` field is a '\0' terminated string which identifies 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 (in 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 offset;
+		__u32 unused;
+		char name[];
+	};