[PATCH] xen/wait: Describe RSB safety

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

It turns out that we do in fact have RSB safety here, but not for obvious
reasons.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
---
 xen/common/wait.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/xen/common/wait.c b/xen/common/wait.c
index e45345ede704..1a3b348a383a 100644
--- a/xen/common/wait.c
+++ b/xen/common/wait.c
@@ -210,6 +210,26 @@ void check_wakeup_from_wait(void)
     }
 
     /*
+     * We are about to jump into a deeper call tree.  In principle, this risks
+     * executing more RET than CALL instructions, and underflowing the RSB.
+     *
+     * However, we are pinned to the same CPU as previously.  Therefore,
+     * either:
+     *
+     *   1) We've scheduled another vCPU in the meantime, and the context
+     *      switch path has (by default) issued IPBP which flushes the RSB, or
+     *
+     *   2) We're still in the same context.  Returning back to the deeper
+     *      call tree is resuming the execution path we left, and remains
+     *      balanced as far as that logic is concerned.
+     *
+     *      In fact, the path though the scheduler will execute more CALL than
+     *      RET instructions, making the RSB unbalanced in the safe direction.
+     *
+     * Therefore, no actions are necessary here to maintain RSB safety.
+     */
+
+    /*
      * Hand-rolled longjmp().
      *
      * check_wakeup_from_wait() is always called with a shallow stack,
-- 
2.11.0