KVM: debugfs: expose pid of vcpu threads

Add a new debugfs file to expose the pid of each vcpu threads. This
is very helpful for userland tools to get the vcpu pids without
worrying about thread naming conventions of the VMM.
Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
Message-Id: <20220523190327.2658-1-vineeth@bitbyteword.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/kvm_host.h

@@ -1435,6 +1435,8 @@ int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state);
 
 #ifdef __KVM_HAVE_ARCH_VCPU_DEBUGFS
 void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu, struct dentry *debugfs_dentry);
+#else
+static inline void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu) {}
 #endif
 
 int kvm_arch_hardware_enable(void);

virt/kvm/kvm_main.c

@@ -3822,9 +3822,18 @@ static int create_vcpu_fd(struct kvm_vcpu *vcpu)
 	return anon_inode_getfd(name, &kvm_vcpu_fops, vcpu, O_RDWR | O_CLOEXEC);
 }
 
+#ifdef __KVM_HAVE_ARCH_VCPU_DEBUGFS
+static int vcpu_get_pid(void *data, u64 *val)
+{
+	struct kvm_vcpu *vcpu = (struct kvm_vcpu *) data;
+	*val = pid_nr(rcu_access_pointer(vcpu->pid));
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(vcpu_get_pid_fops, vcpu_get_pid, NULL, "%llu\n");
+
 static void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
 {
-#ifdef __KVM_HAVE_ARCH_VCPU_DEBUGFS
 	struct dentry *debugfs_dentry;
 	char dir_name[ITOA_MAX_LEN * 2];
 
@@ -3834,10 +3843,12 @@ static void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
 	snprintf(dir_name, sizeof(dir_name), "vcpu%d", vcpu->vcpu_id);
 	debugfs_dentry = debugfs_create_dir(dir_name,
 					    vcpu->kvm->debugfs_dentry);
+	debugfs_create_file("pid", 0444, debugfs_dentry, vcpu,
+			    &vcpu_get_pid_fops);
 
 	kvm_arch_create_vcpu_debugfs(vcpu, debugfs_dentry);
-#endif
 }
+#endif
 
 /*
  * Creates some virtual cpus.  Good luck creating more than one.