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kvm: stats: tell userspace which values are boolean

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Some of the statistics values exported by KVM are always only 0 or 1.
It can be useful to export this fact to userspace so that it can track
them specially (for example by polling the value every now and then to
compute a % of time spent in a specific state).

Therefore, add "boolean value" as a new "unit".  While it is not exactly
a unit, it walks and quacks like one.  In particular, using the type
would be wrong because boolean values could be instantaneous or peak
values (e.g. "is the rmap allocated?") or even two-bucket histograms
(e.g. "number of posted vs. non-posted interrupt injections").

Suggested-by: Amneesh Singh <natto@weirdnatto.in>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2022-07-14 07:27:31 -04:00
parent 84e7051c0b
commit 1b870fa557
4 changed files with 18 additions and 2 deletions
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6
Documentation/virt/kvm/api.rst
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@ -5657,6 +5657,7 @@ 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_BOOLEAN (0x4 << KVM_STATS_UNIT_SHIFT)
#define KVM_STATS_UNIT_MAX KVM_STATS_UNIT_CYCLES
#define KVM_STATS_BASE_SHIFT 8
@ -5724,6 +5725,11 @@ Bits 4-7 of ``flags`` encode the unit:
It indicates that the statistics data is used to measure time or latency.
* ``KVM_STATS_UNIT_CYCLES``
It indicates that the statistics data is used to measure CPU clock cycles.
* ``KVM_STATS_UNIT_BOOLEAN``
It indicates that the statistic will always be either 0 or 1. Boolean
statistics of "peak" type will never go back from 1 to 0. Boolean
statistics can be linear histograms (with two buckets) but not logarithmic
histograms.
Bits 8-11 of ``flags``, together with ``exponent``, encode the scale of the
unit:

2
arch/x86/kvm/x86.c
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@ -298,7 +298,7 @@ const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
STATS_DESC_COUNTER(VCPU, directed_yield_successful),
STATS_DESC_COUNTER(VCPU, preemption_reported),
STATS_DESC_COUNTER(VCPU, preemption_other),
STATS_DESC_ICOUNTER(VCPU, guest_mode)
STATS_DESC_IBOOLEAN(VCPU, guest_mode)
};
const struct kvm_stats_header kvm_vcpu_stats_header = {

11
include/linux/kvm_host.h
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@ -1822,6 +1822,15 @@ struct _kvm_stats_desc {
STATS_DESC_PEAK(SCOPE, name, KVM_STATS_UNIT_NONE, \
KVM_STATS_BASE_POW10, 0)
/* Instantaneous boolean value, read only */
#define STATS_DESC_IBOOLEAN(SCOPE, name) \
STATS_DESC_INSTANT(SCOPE, name, KVM_STATS_UNIT_BOOLEAN, \
KVM_STATS_BASE_POW10, 0)
/* Peak (sticky) boolean value, read/write */
#define STATS_DESC_PBOOLEAN(SCOPE, name) \
STATS_DESC_PEAK(SCOPE, name, KVM_STATS_UNIT_BOOLEAN, \
KVM_STATS_BASE_POW10, 0)
/* Cumulative time in nanosecond */
#define STATS_DESC_TIME_NSEC(SCOPE, name) \
STATS_DESC_CUMULATIVE(SCOPE, name, KVM_STATS_UNIT_SECONDS, \
@ -1853,7 +1862,7 @@ struct _kvm_stats_desc {
HALT_POLL_HIST_COUNT), \
STATS_DESC_LOGHIST_TIME_NSEC(VCPU_GENERIC, halt_wait_hist, \
HALT_POLL_HIST_COUNT), \
STATS_DESC_ICOUNTER(VCPU_GENERIC, blocking)
STATS_DESC_IBOOLEAN(VCPU_GENERIC, blocking)
extern struct dentry *kvm_debugfs_dir;

1
include/uapi/linux/kvm.h
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@ -2083,6 +2083,7 @@ struct kvm_stats_header {
#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_BOOLEAN (0x4 << KVM_STATS_UNIT_SHIFT)
#define KVM_STATS_UNIT_MAX KVM_STATS_UNIT_CYCLES
#define KVM_STATS_BASE_SHIFT 8