[RFC PATCH 2/5] rseq: add per-task rseq operation state

[odion@efficios.com]

From: Olivier Dion <odion@efficios.com>

Add the task-side state for rseq operations: the rseq_event::rseq_op
state indicating operation processing is enabled for the task, and the
nr_ops counter in struct rseq_data. nr_ops is edge triggered: its 0<->1
transition enables/disables rseq_event::rseq_op and the user visible
RSEQ_CS_FLAG_RSEQ_OP_ENABLED flag. This makes so that only thread that
enable RSEQ operations through prctl takes a performance hit.

Include rseq_op in the sched-switch raise condition so pending operations
force a return through the rseq exit path, and declare the rseq_op_prctl()
entry point (with a CONFIG_RSEQ=n stub).

Signed-off-by: Olivier Dion <odion@efficios.com>
---
 include/linux/rseq.h       | 10 +++++++++-
 include/linux/rseq_types.h |  9 +++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/rseq.h b/include/linux/rseq.h
index 7ef79b25e714..d1e33906d863 100644
--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -69,7 +69,9 @@ static __always_inline void rseq_sched_switch_event(struct task_struct *t)
 		 * was via interrupt from user space. ev->has_rseq does not have
 		 * to be evaluated here because rseq_v2() implies has_rseq.
 		 */
-		bool raise = ev->user_irq | ev->ids_changed;
+		bool raise = (ev->user_irq |
+			      ev->ids_changed |
+			      ev->rseq_op);
 
 		if (raise) {
 			ev->sched_switch = true;
@@ -172,6 +174,8 @@ static inline unsigned int rseq_alloc_align(void)
 	return 1U << get_count_order(offsetof(struct rseq, end));
 }
 
+int rseq_op_prctl(unsigned long arg2, unsigned long arg3);
+
 #else /* CONFIG_RSEQ */
 static inline bool rseq_v2(struct task_struct *t) { return false; }
 static inline void rseq_handle_slowpath(struct pt_regs *regs) { }
@@ -182,6 +186,10 @@ static inline void rseq_force_update(void) { }
 static inline void rseq_virt_userspace_exit(void) { }
 static inline void rseq_fork(struct task_struct *t, u64 clone_flags) { }
 static inline void rseq_execve(struct task_struct *t) { }
+static inline int rseq_op_prctl(unsigned long arg2, unsigned long arg3)
+{
+	return -ENOTSUPP;
+}
 #endif  /* !CONFIG_RSEQ */
 
 #ifdef CONFIG_DEBUG_RSEQ
diff --git a/include/linux/rseq_types.h b/include/linux/rseq_types.h
index 85739a63e85e..059292695c34 100644
--- a/include/linux/rseq_types.h
+++ b/include/linux/rseq_types.h
@@ -23,6 +23,7 @@ struct rseq;
  *			exit to user
  * @ids_changed:	Indicator that IDs need to be updated
  * @user_irq:		True on interrupt entry from user mode
+ * @rseq_op:		Rseq operation processing is enabled for the task
  * @has_rseq:		Greater than 0 if the task has a rseq pointer installed.
  *			Contains the RSEQ version number
  * @error:		Compound error code for the slow path to analyze
@@ -44,6 +45,7 @@ struct rseq_event {
 					u8	sched_switch;
 					u8	ids_changed;
 					u8	user_irq;
+					u8	rseq_op;
 				};
 			};
 
@@ -115,6 +117,7 @@ struct rseq_slice {
  * @event:	Storage for event management
  * @ids:	Storage for cached CPU ID and MM CID
  * @slice:	Storage for time slice extension data
+ * @nr_ops:	Number of registered rseq operations
  */
 struct rseq_data {
 	struct rseq __user		*usrptr;
@@ -125,6 +128,12 @@ struct rseq_data {
 #ifdef CONFIG_RSEQ_SLICE_EXTENSION
 	struct rseq_slice		slice;
 #endif
+	/*
+	 * Number of rseq operations registered for the task. Edge triggered:
+	 * the 0<->1 transition enables/disables rseq_event::rseq_op and the
+	 * RSEQ_CS_FLAG_RSEQ_OP_ENABLED user flag.
+	 */
+	u32				nr_ops;
 };
 
 #else /* CONFIG_RSEQ */
-- 
2.54.0