[PATCH v6 14/17] powerpc: Add support for setting SPRN_TIDR

[Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>]

We need the SPRN_TIDR to bet set for use with fast thread-wakeup
(core-to-core wakeup).  Each thread in a process needs to have a
unique id within the process but as explained below, for now, we
assign globally unique thread ids to all threads in the system.

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/processor.h |  4 ++
 arch/powerpc/kernel/process.c        | 74 ++++++++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index fab7ff8..bf6ba63 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h
@@ -232,6 +232,10 @@ struct debug_reg {
 struct thread_struct {
 	unsigned long	ksp;		/* Kernel stack pointer */
 
+#ifdef CONFIG_PPC_VAS
+	unsigned long	tidr;
+#endif
+
 #ifdef CONFIG_PPC64
 	unsigned long	ksp_vsid;
 #endif
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 9f3e2c9..6123859 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1213,6 +1213,16 @@ struct task_struct *__switch_to(struct task_struct *prev,
 		hard_irq_disable();
 	}
 
+#ifdef CONFIG_PPC_VAS
+	mtspr(SPRN_TIDR, new->thread.tidr);
+#endif
+	/*
+	 * We can't take a PMU exception inside _switch() since there is a
+	 * window where the kernel stack SLB and the kernel stack are out
+	 * of sync. Hard disable here.
+	 */
+	hard_irq_disable();
+
 	/*
 	 * Call restore_sprs() before calling _switch(). If we move it after
 	 * _switch() then we miss out on calling it for new tasks. The reason
@@ -1449,9 +1459,70 @@ void flush_thread(void)
 #endif /* CONFIG_HAVE_HW_BREAKPOINT */
 }
 
+#ifdef CONFIG_PPC_VAS
+static DEFINE_SPINLOCK(vas_thread_id_lock);
+static DEFINE_IDA(vas_thread_ida);
+
+/*
+ * We need to assign an unique thread id to each thread in a process. This
+ * thread id is intended to be used with the Fast Thread-wakeup (aka Core-
+ * to-core wakeup) mechanism being implemented on top of Virtual Accelerator
+ * Switchboard (VAS).
+ *
+ * To get a unique thread-id per process we could simply use task_pid_nr()
+ * but the problem is that task_pid_nr() is not yet available for the thread
+ * when copy_thread() is called. Fixing that would require changing more
+ * intrusive arch-neutral code in code path in copy_process()?.
+ *
+ * Further, to assign unique thread ids within each process, we need an
+ * atomic field (or an IDR) in task_struct, which again intrudes into the
+ * arch-neutral code.
+ *
+ * So try to assign globally unique thraed ids for now.
+ */
+static int assign_thread_id(void)
+{
+	int index;
+	int err;
+
+again:
+	if (!ida_pre_get(&vas_thread_ida, GFP_KERNEL))
+		return -ENOMEM;
+
+	spin_lock(&vas_thread_id_lock);
+	err = ida_get_new_above(&vas_thread_ida, 1, &index);
+	spin_unlock(&vas_thread_id_lock);
+
+	if (err == -EAGAIN)
+		goto again;
+	else if (err)
+		return err;
+
+	if (index > MAX_USER_CONTEXT) {
+		spin_lock(&vas_thread_id_lock);
+		ida_remove(&vas_thread_ida, index);
+		spin_unlock(&vas_thread_id_lock);
+		return -ENOMEM;
+	}
+
+	return index;
+}
+
+static void free_thread_id(int id)
+{
+	spin_lock(&vas_thread_id_lock);
+	ida_remove(&vas_thread_ida, id);
+	spin_unlock(&vas_thread_id_lock);
+}
+#endif /* CONFIG_PPC_VAS */
+
+
 void
 release_thread(struct task_struct *t)
 {
+#ifdef CONFIG_PPC_VAS
+	free_thread_id(t->thread.tidr);
+#endif
 }
 
 /*
@@ -1587,6 +1658,9 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
 #endif
 
 	setup_ksp_vsid(p, sp);
+#ifdef CONFIG_PPC_VAS
+	p->thread.tidr = assign_thread_id();
+#endif
 
 #ifdef CONFIG_PPC64 
 	if (cpu_has_feature(CPU_FTR_DSCR)) {
-- 
2.7.4