[RFC PATCH 5/6] powerpc/pseries: Introduce helpers to gatekeep DTLB usage

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

Since we would be introducing a new user of the DTL buffer in a
subsequent patch, add helpers to gatekeep use of the DTL buffer. The
current usage of the DTL buffer from debugfs is at a per-cpu level
(corresponding to the cpu debugfs file that is opened). Subsequently, we
will have users enabling/accessing DTLB for all online cpus. These
helpers allow any number of per-cpu users, or a single global user
exclusively.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 arch/powerpc/include/asm/plpar_wrappers.h |  2 ++
 arch/powerpc/platforms/pseries/dtl.c      | 10 ++++++-
 arch/powerpc/platforms/pseries/lpar.c     | 36 +++++++++++++++++++++++
 3 files changed, 47 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/plpar_wrappers.h b/arch/powerpc/include/asm/plpar_wrappers.h
index d08feb1bc2bd..ab7dd454b6eb 100644
--- a/arch/powerpc/include/asm/plpar_wrappers.h
+++ b/arch/powerpc/include/asm/plpar_wrappers.h
@@ -88,6 +88,8 @@ static inline long register_dtl(unsigned long cpu, unsigned long vpa)
 	return vpa_call(H_VPA_REG_DTL, cpu, vpa);
 }
 
+extern bool register_dtl_buffer_access(bool global);
+extern void unregister_dtl_buffer_access(bool global);
 extern void register_dtl_buffer(int cpu);
 extern void alloc_dtl_buffers(void);
 extern void vpa_init(int cpu);
diff --git a/arch/powerpc/platforms/pseries/dtl.c b/arch/powerpc/platforms/pseries/dtl.c
index fb05804adb2f..dd28296c9903 100644
--- a/arch/powerpc/platforms/pseries/dtl.c
+++ b/arch/powerpc/platforms/pseries/dtl.c
@@ -193,11 +193,15 @@ static int dtl_enable(struct dtl *dtl)
 	if (dtl->buf)
 		return -EBUSY;
 
+	if (register_dtl_buffer_access(false))
+		return -EBUSY;
+
 	n_entries = dtl_buf_entries;
 	buf = kmem_cache_alloc_node(dtl_cache, GFP_KERNEL, cpu_to_node(dtl->cpu));
 	if (!buf) {
 		printk(KERN_WARNING "%s: buffer alloc failed for cpu %d\n",
 				__func__, dtl->cpu);
+		unregister_dtl_buffer_access(false);
 		return -ENOMEM;
 	}
 
@@ -214,8 +218,11 @@ static int dtl_enable(struct dtl *dtl)
 	}
 	spin_unlock(&dtl->lock);
 
-	if (rc)
+	if (rc) {
+		unregister_dtl_buffer_access(false);
 		kmem_cache_free(dtl_cache, buf);
+	}
+
 	return rc;
 }
 
@@ -227,6 +234,7 @@ static void dtl_disable(struct dtl *dtl)
 	dtl->buf = NULL;
 	dtl->buf_entries = 0;
 	spin_unlock(&dtl->lock);
+	unregister_dtl_buffer_access(false);
 }
 
 /* file interface */
diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
index 3375ca8cefb5..6af5a2a11deb 100644
--- a/arch/powerpc/platforms/pseries/lpar.c
+++ b/arch/powerpc/platforms/pseries/lpar.c
@@ -65,6 +65,42 @@ EXPORT_SYMBOL(plpar_hcall);
 EXPORT_SYMBOL(plpar_hcall9);
 EXPORT_SYMBOL(plpar_hcall_norets);
 
+static DEFINE_SPINLOCK(dtl_buffer_refctr_lock);
+static unsigned int dtl_buffer_global_refctr, dtl_buffer_percpu_refctr;
+
+bool register_dtl_buffer_access(bool global)
+{
+	int rc = 0;
+
+	spin_lock(&dtl_buffer_refctr_lock);
+
+	if ((global && (dtl_buffer_global_refctr || dtl_buffer_percpu_refctr))
+			|| (!global && dtl_buffer_global_refctr)) {
+		rc = -1;
+	} else {
+		if (global)
+			dtl_buffer_global_refctr++;
+		else
+			dtl_buffer_percpu_refctr++;
+	}
+
+	spin_unlock(&dtl_buffer_refctr_lock);
+
+	return rc;
+}
+
+void unregister_dtl_buffer_access(bool global)
+{
+	spin_lock(&dtl_buffer_refctr_lock);
+
+	if (global)
+		dtl_buffer_global_refctr--;
+	else
+		dtl_buffer_percpu_refctr--;
+
+	spin_unlock(&dtl_buffer_refctr_lock);
+}
+
 void alloc_dtl_buffers(void)
 {
 	int cpu;
-- 
2.21.0