KVM: stats: Update doc for histogram statistics

Add documentations for linear and logarithmic histogram statistics.
Signed-off-by: Jing Zhang <jingzhangos@google.com>
Message-Id: <20210802165633.1866976-3-jingzhangos@google.com>
[Small changes to the phrasing. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -5207,6 +5207,9 @@ by a string of size ``name_size``.
 	#define KVM_STATS_TYPE_CUMULATIVE	(0x0 << KVM_STATS_TYPE_SHIFT)
 	#define KVM_STATS_TYPE_INSTANT		(0x1 << KVM_STATS_TYPE_SHIFT)
 	#define KVM_STATS_TYPE_PEAK		(0x2 << KVM_STATS_TYPE_SHIFT)
+	#define KVM_STATS_TYPE_LINEAR_HIST	(0x3 << KVM_STATS_TYPE_SHIFT)
+	#define KVM_STATS_TYPE_LOG_HIST		(0x4 << KVM_STATS_TYPE_SHIFT)
+	#define KVM_STATS_TYPE_MAX		KVM_STATS_TYPE_LOG_HIST
 
 	#define KVM_STATS_UNIT_SHIFT		4
 	#define KVM_STATS_UNIT_MASK		(0xF << KVM_STATS_UNIT_SHIFT)
@@ -5214,18 +5217,20 @@ by a string of size ``name_size``.
 	#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;
+		__u32 bucket_size;
 		char name[];
 	};
 
@@ -5235,21 +5240,35 @@ 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.
+    The statistics reports a cumulative count. The value of data can only be increased.
     Most of the counters used in KVM are of this type.
     The corresponding ``size`` field for this type is always 1.
     All cumulative statistics data are read/write.
   * ``KVM_STATS_TYPE_INSTANT``
-    The statistics data is instantaneous. Its value can be increased or
+    The statistics reports an instantaneous value. 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.
     All instant statistics are read only.
     The corresponding ``size`` field for this type is always 1.
   * ``KVM_STATS_TYPE_PEAK``
-    The statistics data is peak. The value of data can only be increased, and
-    represents a peak value for a measurement, for example the maximum number
+    The statistics data reports a peak value, for example the maximum number
     of items in a hash table bucket, the longest time waited and so on.
+    The value of data can only be increased.
     The corresponding ``size`` field for this type is always 1.
+  * ``KVM_STATS_TYPE_LINEAR_HIST``
+    The statistic is reported as a linear histogram. The number of
+    buckets is specified by the ``size`` field. The size of buckets is specified
+    by the ``hist_param`` field. The range of the Nth bucket (1 <= N < ``size``)
+    is [``hist_param``*(N-1), ``hist_param``*N), while the range of the last
+    bucket is [``hist_param``*(``size``-1), +INF). (+INF means positive infinity
+    value.) The bucket value indicates how many samples fell in the bucket's range.
+  * ``KVM_STATS_TYPE_LOG_HIST``
+    The statistic is reported as a logarithmic histogram. The number of
+    buckets is specified by the ``size`` field. The range of the first bucket is
+    [0, 1), while the range of the last bucket is [pow(2, ``size``-2), +INF).
+    Otherwise, The Nth bucket (1 < N < ``size``) covers
+    [pow(2, N-2), pow(2, N-1)). The bucket value indicates how many samples fell
+    in the bucket's range.
 
 Bits 4-7 of ``flags`` encode the unit:
   * ``KVM_STATS_UNIT_NONE``
@@ -5282,9 +5301,9 @@ unsigned 64bit data.
 The ``offset`` field is the offset from the start of Data Block to the start of
 the corresponding statistics data.
 
-The ``unused`` field is reserved for future support for other types of
-statistics data, like log/linear histogram. Its value is always 0 for the types
-defined above.
+The ``bucket_size`` field is used as a parameter for histogram statistics data.
+It is only used by linear histogram statistics data, specifying the size of a
+bucket.
 
 The ``name`` field is the name string of the statistics data. The name string
 starts at the end of ``struct kvm_stats_desc``.  The maximum length including