core, x86: Add user return notifiers

Add a general per-cpu notifier that is called whenever the kernel is
about to return to userspace.  The notifier uses a thread_info flag
and existing checks, so there is no impact on user return or context
switch fast paths.

This will be used initially to speed up KVM task switching by lazily
updating MSRs.

Signed-off-by: Avi Kivity <avi@redhat.com>
LKML-Reference: <1253342422-13811-1-git-send-email-avi@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/include/linux/user-return-notifier.h b/include/linux/user-return-notifier.h
new file mode 100644
index 000000000000..b6ac056291d7
--- /dev/null
+++ b/include/linux/user-return-notifier.h
@@ -0,0 +1,42 @@
+#ifndef _LINUX_USER_RETURN_NOTIFIER_H
+#define _LINUX_USER_RETURN_NOTIFIER_H
+
+#ifdef CONFIG_USER_RETURN_NOTIFIER
+
+#include <linux/list.h>
+#include <linux/sched.h>
+
+struct user_return_notifier {
+	void (*on_user_return)(struct user_return_notifier *urn);
+	struct hlist_node link;
+};
+
+
+void user_return_notifier_register(struct user_return_notifier *urn);
+void user_return_notifier_unregister(struct user_return_notifier *urn);
+
+static inline void propagate_user_return_notify(struct task_struct *prev,
+						struct task_struct *next)
+{
+	if (test_tsk_thread_flag(prev, TIF_USER_RETURN_NOTIFY)) {
+		clear_tsk_thread_flag(prev, TIF_USER_RETURN_NOTIFY);
+		set_tsk_thread_flag(next, TIF_USER_RETURN_NOTIFY);
+	}
+}
+
+void fire_user_return_notifiers(void);
+
+#else
+
+struct user_return_notifier {};
+
+static inline void propagate_user_return_notify(struct task_struct *prev,
+						struct task_struct *next)
+{
+}
+
+static inline void fire_user_return_notifiers(void) {}
+
+#endif
+
+#endif