kvm: stats: tell userspace which values are boolean

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>

Author: bonzini

Documentation/virt/kvm/api.rst

@@ -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:

arch/x86/kvm/x86.c

@@ -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 = {

include/linux/kvm_host.h

@@ -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;
 

include/uapi/linux/kvm.h

@@ -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