Re: [patch V2 36/37] rseq: Switch to TIF_RSEQ if supported

[Sean Christopherson <seanjc@google.com>]

On Sat, Aug 23, 2025, Thomas Gleixner wrote:
> @@ -122,7 +122,7 @@ static inline void rseq_force_update(voi
>   */
>  static inline void rseq_virt_userspace_exit(void)
>  {
> -	if (current->rseq_event.sched_switch)
> +	if (!IS_ENABLED(CONFIG_HAVE_GENERIC_TIF_BITS) && current->rseq_event.sched_switch)

Rather than pivot on CONFIG_HAVE_GENERIC_TIF_BITS, which makes the "why" quite
difficult to find/understand, what if this checks TIF_RSEQ == TIF_NOTIFY_RESUME?
That would also allow architectures to define TIF_RSEQ without switching to the
generic TIF bits implementation (though I don't know that we want to encourage
that?).

Updating the comment to explain what's going on would also be helpful, e.g.

diff --git a/include/linux/rseq.h b/include/linux/rseq.h
index 185a4875b261..9a8e238ae9d1 100644
--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -112,17 +112,17 @@ static inline void rseq_force_update(void)
 
 /*
  * KVM/HYPERV invoke resume_user_mode_work() before entering guest mode,
- * which clears TIF_NOTIFY_RESUME. To avoid updating user space RSEQ in
- * that case just to do it eventually again before returning to user space,
- * the entry resume_user_mode_work() invocation is ignored as the register
- * argument is NULL.
+ * which clears TIF_NOTIFY_RESUME on architectures that don't provide a separate
+ * TIF_RSEQ flag. To avoid updating user space RSEQ in that case just to do it
+ * eventually again before returning to user space, __rseq_handle_slowpath()
+ * does nothing when invoked with NULL register state.
  *
- * After returning from guest mode, they have to invoke this function to
- * re-raise TIF_NOTIFY_RESUME if necessary.
+ * After returning from guest mode, before exiting to userspace, hypervisors
+ * must invoke this function to re-raise TIF_NOTIFY_RESUME if necessary.
  */
 static inline void rseq_virt_userspace_exit(void)
 {
-       if (!IS_ENABLED(CONFIG_HAVE_GENERIC_TIF_BITS) && current->rseq_event.sched_switch)
+       if (TIF_RSEQ == TIF_NOTIFY_RESUME && current->rseq_event.sched_switch)
                rseq_raise_notify_resume(current);
 }
 
>  		rseq_raise_notify_resume(current);
>  }