On Thu, Jun 17, 2021 at 04:41:44AM +0000, Jing Zhang wrote:
+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];- };
So you have no idea the size of the whole header when reading it? That feels odd, are you sure it's not needed?
+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".
Why do you have "name_size" but not "id_size"?
And is this a \0 terminated string? If so, please state it here.
And what is the max size of this string?
And again, should it be [], not [0]?
Will the header be padded out to any specific byte boundry (4/8/32/whatever) before the other headers?
+The ``name_size`` field is the size (byte) of the statistics name string
s/byte/in bytes/
+(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[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`` field 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 8-11 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.
Might also want to show a negative example here for exponent, like you show above for time.
+The ``size`` field is the number of values (u64) of this statistics data. Its +value is usually 1 for most of simple statistics.
What does "u64" mean here?
thanks,
greg k-h