Merge tag 'x86-cpu-2021-08-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 cache flush updates from Thomas Gleixner:
 "A reworked version of the opt-in L1D flush mechanism.

  This is a stop gap for potential future speculation related hardware
  vulnerabilities and a mechanism for truly security paranoid
  applications.

  It allows a task to request that the L1D cache is flushed when the
  kernel switches to a different mm. This can be requested via prctl().

  Changes vs the previous versions:

   - Get rid of the software flush fallback

   - Make the handling consistent with other mitigations

   - Kill the task when it ends up on a SMT enabled core which defeats
     the purpose of L1D flushing obviously"

* tag 'x86-cpu-2021-08-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  Documentation: Add L1D flushing Documentation
  x86, prctl: Hook L1D flushing in via prctl
  x86/mm: Prepare for opt-in based L1D flush in switch_mm()
  x86/process: Make room for TIF_SPEC_L1D_FLUSH
  sched: Add task_work callback for paranoid L1D flush
  x86/mm: Refactor cond_ibpb() to support other use cases
  x86/smp: Add a per-cpu view of SMT state

Documentation/userspace-api/spec_ctrl.rst

@@ -106,3 +106,11 @@ Speculation misfeature controls
    * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_ENABLE, 0, 0);
    * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_DISABLE, 0, 0);
    * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_INDIRECT_BRANCH, PR_SPEC_FORCE_DISABLE, 0, 0);
+
+- PR_SPEC_L1D_FLUSH: Flush L1D Cache on context switch out of the task
+                        (works only when tasks run on non SMT cores)
+
+  Invocations:
+   * prctl(PR_GET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, 0, 0, 0);
+   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_ENABLE, 0, 0);
+   * prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_L1D_FLUSH, PR_SPEC_DISABLE, 0, 0);