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, 10 insertions, 0 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index 7f418bbc261a..4e312fffbfd7 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -83,6 +83,13 @@ config KRETPROBES
         def_bool y
         depends on KPROBES && HAVE_KRETPROBES
 
+config USER_RETURN_NOTIFIER
+        bool
+        depends on HAVE_USER_RETURN_NOTIFIER
+        help
+          Provide a kernel-internal notification when a cpu is about to
+          switch to user mode.
+
 config HAVE_IOREMAP_PROT
         bool
 
@@ -126,4 +133,7 @@ config HAVE_DMA_API_DEBUG
 config HAVE_DEFAULT_NO_SPIN_MUTEXES
         bool
 
+config HAVE_USER_RETURN_NOTIFIER
+        bool
+
 source "kernel/gcov/Kconfig"