Re: [PATCH v1 2/4] KVM: stats: Update doc for histogram statistics

[David Matlack <dmatlack@google.com>]

On Tue, Jul 06, 2021 at 06:03:48PM +0000, Jing Zhang wrote:
> Add documentations for linear and logarithmic histogram statistics.
> Add binary stats capability text which is missing during merge of
> the binary stats patch.
> 
> Signed-off-by: Jing Zhang <jingzhangos@google.com>
> ---
>  Documentation/virt/kvm/api.rst | 36 +++++++++++++++++++++++++++++++---
>  1 file changed, 33 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 3b6e3b1628b4..948d33c26704 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -5171,6 +5171,9 @@ 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)
> @@ -5178,11 +5181,13 @@ 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

Should these FOO_MAX additions go in a separate commit too?

>  
>  	struct kvm_stats_desc {
>  		__u32 flags;
> @@ -5214,6 +5219,22 @@ Bits 0-3 of ``flags`` encode the type:
>      represents a peak value for a measurement, for example the maximum number
>      of items in a hash table bucket, the longest time waited and so on.
>      The corresponding ``size`` field for this type is always 1.
> +  * ``KVM_STATS_TYPE_LINEAR_HIST``
> +    The statistics data is in the form of 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``)

Using 1-based indexes is a little jarring but maybe that's just me :).

> +    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 times the statistics data is in
> +    the bucket's range.
> +  * ``KVM_STATS_TYPE_LOG_HIST``
> +    The statistics data is in the form of logarithmic histogram. The number of
> +    buckets is specified by the ``size`` field. The base of logarithm is
> +    specified by the ``hist_param`` field. The range of the Nth bucket (1 < N <

Should 1 be 2 here? The first bucket uses a slightly differen formula as
you mention in the next sentence.

Actually it may be clearer if you re-ordered the sentences a bit:

  The range of the first bucket is [0, 1), while the range of the last
  bucket is [pow(hist_param, size-1), +INF). Otherwise the Nth bucket
  (1-indexed) covers [pow(hist_param, N-2), pow(histparam, N-1)).

> +    ``size``) is [pow(``hist_param``, N-2), pow(``hist_param``, N-1)). The range
> +    of the first bucket is [0, 1), while the range of the last bucket is
> +    [pow(``hist_param``, ``size``-2), +INF). The bucket value indicates how many
> +    times the statistics data is in the bucket's range.
>  
>  Bits 4-7 of ``flags`` encode the unit:
>    * ``KVM_STATS_UNIT_NONE``
> @@ -5246,9 +5267,10 @@ 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 ``hist_param`` field is used as a parameter for histogram statistics data.
> +For linear histogram statistics data, it indicates the size of a bucket. For
> +logarithmic histogram statistics data, it indicates the base of the logarithm.
> +Only base of 2 is supported fo logarithmic histogram.

s/fo/for/

>  
>  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
> @@ -7182,3 +7204,11 @@ The argument to KVM_ENABLE_CAP is also a bitmask, and must be a subset
>  of the result of KVM_CHECK_EXTENSION.  KVM will forward to userspace
>  the hypercalls whose corresponding bit is in the argument, and return
>  ENOSYS for the others.
> +
> +8.35 KVM_CAP_STATS_BINARY_FD
> +----------------------------
> +
> +:Architectures: all
> +
> +This capability indicates the feature that userspace can get a file descriptor
> +for every VM and VCPU to read statistics data in binary format.
> -- 
> 2.32.0.93.g670b81a890-goog
>