kvm: Add helper function for creating VM worker threads

Add a function to create a kernel thread associated with a given VM. In
particular, it ensures that the worker thread inherits the priority and
cgroups of the calling thread.

Signed-off-by: Junaid Shahid <junaids@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 90 insertions, 0 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 719fc3e15ea4..52ed5f66e8f9 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1382,4 +1382,10 @@ static inline int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
 }
 #endif /* CONFIG_HAVE_KVM_VCPU_RUN_PID_CHANGE */
 
+typedef int (*kvm_vm_thread_fn_t)(struct kvm *kvm, uintptr_t data);
+
+int kvm_vm_create_worker_thread(struct kvm *kvm, kvm_vm_thread_fn_t thread_fn,
+                                uintptr_t data, const char *name,
+                                struct task_struct **thread_ptr);
+
 #endif
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d6f0696d98ef..8aed32b604d9 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -50,6 +50,7 @@
 #include <linux/bsearch.h>
 #include <linux/io.h>
 #include <linux/lockdep.h>
+#include <linux/kthread.h>
 
 #include <asm/processor.h>
 #include <asm/ioctl.h>
@@ -4371,3 +4372,86 @@ void kvm_exit(void)
         kvm_vfio_ops_exit();
 }
 EXPORT_SYMBOL_GPL(kvm_exit);
+
+struct kvm_vm_worker_thread_context {
+        struct kvm *kvm;
+        struct task_struct *parent;
+        struct completion init_done;
+        kvm_vm_thread_fn_t thread_fn;
+        uintptr_t data;
+        int err;
+};
+
+static int kvm_vm_worker_thread(void *context)
+{
+        /*
+         * The init_context is allocated on the stack of the parent thread, so
+         * we have to locally copy anything that is needed beyond initialization
+         */
+        struct kvm_vm_worker_thread_context *init_context = context;
+        struct kvm *kvm = init_context->kvm;
+        kvm_vm_thread_fn_t thread_fn = init_context->thread_fn;
+        uintptr_t data = init_context->data;
+        int err;
+
+        err = kthread_park(current);
+        /* kthread_park(current) is never supposed to return an error */
+        WARN_ON(err != 0);
+        if (err)
+                goto init_complete;
+
+        err = cgroup_attach_task_all(init_context->parent, current);
+        if (err) {
+                kvm_err("%s: cgroup_attach_task_all failed with err %d\n",
+                        __func__, err);
+                goto init_complete;
+        }
+
+        set_user_nice(current, task_nice(init_context->parent));
+
+init_complete:
+        init_context->err = err;
+        complete(&init_context->init_done);
+        init_context = NULL;
+
+        if (err)
+                return err;
+
+        /* Wait to be woken up by the spawner before proceeding. */
+        kthread_parkme();
+
+        if (!kthread_should_stop())
+                err = thread_fn(kvm, data);
+
+        return err;
+}
+
+int kvm_vm_create_worker_thread(struct kvm *kvm, kvm_vm_thread_fn_t thread_fn,
+                                uintptr_t data, const char *name,
+                                struct task_struct **thread_ptr)
+{
+        struct kvm_vm_worker_thread_context init_context = {};
+        struct task_struct *thread;
+
+        *thread_ptr = NULL;
+        init_context.kvm = kvm;
+        init_context.parent = current;
+        init_context.thread_fn = thread_fn;
+        init_context.data = data;
+        init_completion(&init_context.init_done);
+
+        thread = kthread_run(kvm_vm_worker_thread, &init_context,
+                             "%s-%d", name, task_pid_nr(current));
+        if (IS_ERR(thread))
+                return PTR_ERR(thread);
+
+        /* kthread_run is never supposed to return NULL */
+        WARN_ON(thread == NULL);
+
+        wait_for_completion(&init_context.init_done);
+
+        if (!init_context.err)
+                *thread_ptr = thread;
+
+        return init_context.err;
+}