Re: [PATCH v9 3/5] KVM: stats: Add documentation for statistics data binary interface

Greg KH
2021-06-16T15:21:23Z

On Mon, Jun 14, 2021 at 09:21:53PM +0000, Jing Zhang wrote:
> Update KVM API documentation for binary statistics.

You should write more here. See my comment at the bottom...

>
> Reviewed-by: David Matlack <dmatlack@google.com>
> Reviewed-by: Ricardo Koller <ricarkol@google.com>
> Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
> Reviewed-by: Fuad Tabba <tabba@google.com>
> Signed-off-by: Jing Zhang <jingzhangos@google.com>
> ---
> Documentation/virt/kvm/api.rst | 177 ++++++++++++++++++++++++++++++++-
> 1 file changed, 176 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index e328caa35d6c..35ee52dbec89 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 +5080,174 @@ 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 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.
> +
> +The ``size`` field is the number of values (u64) of this statistics data. Its
> +value is usually 1 for most of simple statistics.
> +
> +The ``offset`` field is the offset from the start of Data Block to the start of
> +the corresponding statistics data.
> +
> +The ``unused`` 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
> +userspace 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];
> + };

I forgot to comment on this one, sorry for the delay.

Why are you "inventing" your own schema format here for this? Why not
use a well-known or at least well-designed/implemented one that we have
in userspace already?

There are a few that I would love to see in the kernel, varlink being
the best example. We have kernel examples of this and I would consider
using that as a transport for sysfs-like data in the future, but never
got around to it.

So again, why reinvent the wheel to create a custom api when you could
use an existing one?

thanks,

greg k-h