x86: pvclock: Really remove the sched notifier for cross-cpu migrations

This reverts commits 0a4e6be9ca17c54817cf814b4b5aa60478c6df27
and 80f7fdb1c7f0f9266421f823964fd1962681f6ce.

The task migration notifier was originally introduced in order to support
the pvclock vsyscall with non-synchronized TSC, but KVM only supports it
with synchronized TSC.  Hence, on KVM the race condition is only needed
due to a bad implementation on the host side, and even then it's so rare
that it's mostly theoretical.

As far as KVM is concerned it's possible to fix the host, avoiding the
additional complexity in the vDSO and the (re)introduction of the task
migration notifier.

Xen, on the other hand, hasn't yet implemented vsyscall support at
all, so we do not care about its plans for non-synchronized TSC.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 0 insertions, 8 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8222ae40ecb0..26a2e6122734 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -175,14 +175,6 @@ extern void get_iowait_load(unsigned long *nr_waiters, unsigned long *load);
 extern void calc_global_load(unsigned long ticks);
 extern void update_cpu_load_nohz(void);
 
-/* Notifier for when a task gets migrated to a new CPU */
-struct task_migration_notifier {
-        struct task_struct *task;
-        int from_cpu;
-        int to_cpu;
-};
-extern void register_task_migration_notifier(struct notifier_block *n);
-
 extern unsigned long get_parent_ip(unsigned long addr);
 
 extern void dump_cpu_task(int cpu);