Merge tag 'core-entry-2020-12-14' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull core entry/exit updates from Thomas Gleixner:
 "A set of updates for entry/exit handling:

   - More generalization of entry/exit functionality

   - The consolidation work to reclaim TIF flags on x86 and also for
     non-x86 specific TIF flags which are solely relevant for syscall
     related work and have been moved into their own storage space. The
     x86 specific part had to be merged in to avoid a major conflict.

   - The TIF_NOTIFY_SIGNAL work which replaces the inefficient signal
     delivery mode of task work and results in an impressive performance
     improvement for io_uring. The non-x86 consolidation of this is
     going to come seperate via Jens.

   - The selective syscall redirection facility which provides a clean
     and efficient way to support the non-Linux syscalls of WINE by
     catching them at syscall entry and redirecting them to the user
     space emulation. This can be utilized for other purposes as well
     and has been designed carefully to avoid overhead for the regular
     fastpath. This includes the core changes and the x86 support code.

   - Simplification of the context tracking entry/exit handling for the
     users of the generic entry code which guarantee the proper ordering
     and protection.

   - Preparatory changes to make the generic entry code accomodate S390
     specific requirements which are mostly related to their syscall
     restart mechanism"

* tag 'core-entry-2020-12-14' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (36 commits)
  entry: Add syscall_exit_to_user_mode_work()
  entry: Add exit_to_user_mode() wrapper
  entry_Add_enter_from_user_mode_wrapper
  entry: Rename exit_to_user_mode()
  entry: Rename enter_from_user_mode()
  docs: Document Syscall User Dispatch
  selftests: Add benchmark for syscall user dispatch
  selftests: Add kselftest for syscall user dispatch
  entry: Support Syscall User Dispatch on common syscall entry
  kernel: Implement selective syscall userspace redirection
  signal: Expose SYS_USER_DISPATCH si_code type
  x86: vdso: Expose sigreturn address on vdso to the kernel
  MAINTAINERS: Add entry for common entry code
  entry: Fix boot for !CONFIG_GENERIC_ENTRY
  x86: Support HAVE_CONTEXT_TRACKING_OFFSTACK
  context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING_OFFSTACK archs
  sched: Detect call to schedule from critical entry code
  context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK
  context_tracking: Introduce HAVE_CONTEXT_TRACKING_OFFSTACK
  x86: Reclaim unused x86 TI flags
  ...

1 files changed, 17 insertions, 0 deletions

diff --git a/arch/Kconfig b/arch/Kconfig
index ba4e966484ab..7a3371d28508 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -618,6 +618,23 @@ config HAVE_CONTEXT_TRACKING
 	  protected inside rcu_irq_enter/rcu_irq_exit() but preemption or signal
 	  handling on irq exit still need to be protected.
 
+config HAVE_CONTEXT_TRACKING_OFFSTACK
+	bool
+	help
+	  Architecture neither relies on exception_enter()/exception_exit()
+	  nor on schedule_user(). Also preempt_schedule_notrace() and
+	  preempt_schedule_irq() can't be called in a preemptible section
+	  while context tracking is CONTEXT_USER. This feature reflects a sane
+	  entry implementation where the following requirements are met on
+	  critical entry code, ie: before user_exit() or after user_enter():
+
+	  - Critical entry code isn't preemptible (or better yet:
+	    not interruptible).
+	  - No use of RCU read side critical sections, unless rcu_nmi_enter()
+	    got called.
+	  - No use of instrumentation, unless instrumentation_begin() got
+	    called.
+
 config HAVE_TIF_NOHZ
 	bool
 	help