[patch V3 14/37] 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>

---
 include/linux/rseq.h        |    3 +++
 include/linux/rseq_types.h  |   19 +++++++++++++++++++
 include/linux/sched.h       |    1 +
 include/trace/events/rseq.h |    4 ++--
 kernel/rseq.c               |    4 ++++
 5 files changed, 29 insertions(+), 2 deletions(-)

--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -64,11 +64,13 @@ static inline void rseq_fork(struct task
 		t->rseq = NULL;
 		t->rseq_len = 0;
 		t->rseq_sig = 0;
+		t->rseq_ids.cpu_cid = ~0ULL;
 		t->rseq_event.all = 0;
 	} else {
 		t->rseq = current->rseq;
 		t->rseq_len = current->rseq_len;
 		t->rseq_sig = current->rseq_sig;
+		t->rseq_ids.cpu_cid = ~0ULL;
 		t->rseq_event = current->rseq_event;
 	}
 }
@@ -78,6 +80,7 @@ static inline void rseq_execve(struct ta
 	t->rseq = NULL;
 	t->rseq_len = 0;
 	t->rseq_sig = 0;
+	t->rseq_ids.cpu_cid = ~0ULL;
 	t->rseq_event.all = 0;
 }
 
--- a/include/linux/rseq_types.h
+++ b/include/linux/rseq_types.h
@@ -27,4 +27,23 @@ 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;
+		};
+	};
+};
+
 #endif
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1406,6 +1406,7 @@ struct task_struct {
 	u32				rseq_len;
 	u32				rseq_sig;
 	struct rseq_event		rseq_event;
+	struct rseq_ids			rseq_ids;
 # ifdef CONFIG_DEBUG_RSEQ
 	/*
 	 * This is a place holder to save a copy of the rseq fields for
--- 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