[patch V6 14/31] rseq: Cache CPU ID and MM CID values

[Thomas Gleixner <tglx@linutronix.de>]

In preparation for rewriting RSEQ exit to user space handling provide
storage to cache the CPU ID and MM CID values which were written to user
space. That prepares for a quick check, which avoids the update when
nothing changed.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

---
 include/linux/rseq.h        |    7 +++++--
 include/linux/rseq_types.h  |   21 +++++++++++++++++++++
 include/trace/events/rseq.h |    4 ++--
 kernel/rseq.c               |    4 ++++
 4 files changed, 32 insertions(+), 4 deletions(-)
--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -57,6 +57,7 @@ static inline void rseq_virt_userspace_e
 static inline void rseq_reset(struct task_struct *t)
 {
 	memset(&t->rseq, 0, sizeof(t->rseq));
+	t->rseq.ids.cpu_cid = ~0ULL;
 }
 
 static inline void rseq_execve(struct task_struct *t)
@@ -70,10 +71,12 @@ static inline void rseq_execve(struct ta
  */
 static inline void rseq_fork(struct task_struct *t, u64 clone_flags)
 {
-	if (clone_flags & CLONE_VM)
+	if (clone_flags & CLONE_VM) {
 		rseq_reset(t);
-	else
+	} else {
 		t->rseq = current->rseq;
+		t->rseq.ids.cpu_cid = ~0ULL;
+	}
 }
 
 #else /* CONFIG_RSEQ */
--- a/include/linux/rseq_types.h
+++ b/include/linux/rseq_types.h
@@ -31,17 +31,38 @@ struct rseq_event {
 };
 
 /**
+ * struct rseq_ids - Cache for ids, which need to be updated
+ * @cpu_cid:	Compound of @cpu_id and @mm_cid to make the
+ *		compiler emit a single compare on 64-bit
+ * @cpu_id:	The CPU ID which was written last to user space
+ * @mm_cid:	The MM CID which was written last to user space
+ *
+ * @cpu_id and @mm_cid are updated when the data is written to user space.
+ */
+struct rseq_ids {
+	union {
+		u64		cpu_cid;
+		struct {
+			u32	cpu_id;
+			u32	mm_cid;
+		};
+	};
+};
+
+/**
  * struct rseq_data - Storage for all rseq related data
  * @usrptr:	Pointer to the registered user space RSEQ memory
  * @len:	Length of the RSEQ region
  * @sig:	Signature of critial section abort IPs
  * @event:	Storage for event management
+ * @ids:	Storage for cached CPU ID and MM CID
  */
 struct rseq_data {
 	struct rseq __user		*usrptr;
 	u32				len;
 	u32				sig;
 	struct rseq_event		event;
+	struct rseq_ids			ids;
 };
 
 #else /* CONFIG_RSEQ */
--- a/include/trace/events/rseq.h
+++ b/include/trace/events/rseq.h
@@ -21,9 +21,9 @@ TRACE_EVENT(rseq_update,
 	),
 
 	TP_fast_assign(
-		__entry->cpu_id = raw_smp_processor_id();
+		__entry->cpu_id = t->rseq.ids.cpu_id;
 		__entry->node_id = cpu_to_node(__entry->cpu_id);
-		__entry->mm_cid = task_mm_cid(t);
+		__entry->mm_cid = t->rseq.ids.mm_cid;
 	),
 
 	TP_printk("cpu_id=%d node_id=%d mm_cid=%d", __entry->cpu_id,
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -184,6 +184,10 @@ static int rseq_update_cpu_node_id(struc
 	rseq_unsafe_put_user(t, node_id, node_id, efault_end);
 	rseq_unsafe_put_user(t, mm_cid, mm_cid, efault_end);
 
+	/* Cache the user space values */
+	t->rseq.ids.cpu_id = cpu_id;
+	t->rseq.ids.mm_cid = mm_cid;
+
 	/*
 	 * Additional feature fields added after ORIG_RSEQ_SIZE
 	 * need to be conditionally updated only if