KVM: stats: Add stat to detect if vcpu is currently blocking

Add a "blocking" stat that userspace can use to detect the case where a
vCPU is not being run because of an vCPU/guest action, e.g. HLT or WFS on
x86, WFI on arm64, etc...  Current guest/host/halt stats don't show this
well, e.g. if a guest halts for a long period of time then the vCPU could
could appear pathologically blocked due to a host condition, when in
reality the vCPU has been put into a not-runnable state by the guest.

Originally-by: Cannon Matthews <cannonmatthews@google.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Jing Zhang <jingzhangos@google.com>
[sean: renamed stat to "blocking", massaged changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-16-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/kvm_host.h

@@ -1587,7 +1587,8 @@ struct _kvm_stats_desc {
 	STATS_DESC_LOGHIST_TIME_NSEC(VCPU_GENERIC, halt_poll_fail_hist,	       \
 			HALT_POLL_HIST_COUNT),				       \
 	STATS_DESC_LOGHIST_TIME_NSEC(VCPU_GENERIC, halt_wait_hist,	       \
-			HALT_POLL_HIST_COUNT)
+			HALT_POLL_HIST_COUNT),				       \
+	STATS_DESC_ICOUNTER(VCPU_GENERIC, blocking)
 
 extern struct dentry *kvm_debugfs_dir;
 

include/linux/kvm_types.h

@@ -87,6 +87,7 @@ struct kvm_vcpu_stat_generic {
 	u64 halt_poll_success_hist[HALT_POLL_HIST_COUNT];
 	u64 halt_poll_fail_hist[HALT_POLL_HIST_COUNT];
 	u64 halt_wait_hist[HALT_POLL_HIST_COUNT];
+	u64 blocking;
 };
 
 #define KVM_STATS_NAME_SIZE	48

virt/kvm/kvm_main.c

@@ -3282,6 +3282,8 @@ bool kvm_vcpu_block(struct kvm_vcpu *vcpu)
 	struct rcuwait *wait = kvm_arch_vcpu_get_wait(vcpu);
 	bool waited = false;
 
+	vcpu->stat.generic.blocking = 1;
+
 	kvm_arch_vcpu_blocking(vcpu);
 
 	prepare_to_rcuwait(wait);
@@ -3298,6 +3300,8 @@ bool kvm_vcpu_block(struct kvm_vcpu *vcpu)
 
 	kvm_arch_vcpu_unblocking(vcpu);
 
+	vcpu->stat.generic.blocking = 0;
+
 	return waited;
 }