Split init_new_context and destroy_context

For KVM we need to allocate a new context id, but don't really care about
all the mm context around it.

So let's split the alloc and destroy functions for the context id, so we can
grab one without allocating an mm context.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/mmu_context.h

@@ -23,6 +23,8 @@ extern void switch_slb(struct task_struct *tsk, struct mm_struct *mm);
 extern void set_context(unsigned long id, pgd_t *pgd);
 
 #ifdef CONFIG_PPC_BOOK3S_64
+extern int __init_new_context(void);
+extern void __destroy_context(int context_id);
 static inline void mmu_context_init(void) { }
 #else
 extern void mmu_context_init(void);

arch/powerpc/mm/mmu_context_hash64.c

@@ -18,6 +18,7 @@
 #include <linux/mm.h>
 #include <linux/spinlock.h>
 #include <linux/idr.h>
+#include <linux/module.h>
 
 #include <asm/mmu_context.h>
 
@@ -32,7 +33,7 @@ static DEFINE_IDR(mmu_context_idr);
 #define NO_CONTEXT	0
 #define MAX_CONTEXT	((1UL << 19) - 1)
 
-int init_new_context(struct task_struct *tsk, struct mm_struct *mm)
+int __init_new_context(void)
 {
 	int index;
 	int err;
@@ -57,6 +58,18 @@ again:
 		return -ENOMEM;
 	}
 
+	return index;
+}
+EXPORT_SYMBOL_GPL(__init_new_context);
+
+int init_new_context(struct task_struct *tsk, struct mm_struct *mm)
+{
+	int index;
+
+	index = __init_new_context();
+	if (index < 0)
+		return index;
+
 	/* The old code would re-promote on fork, we don't do that
 	 * when using slices as it could cause problem promoting slices
 	 * that have been forced down to 4K
@@ -68,11 +81,16 @@ again:
 	return 0;
 }
 
-void destroy_context(struct mm_struct *mm)
+void __destroy_context(int context_id)
 {
 	spin_lock(&mmu_context_lock);
-	idr_remove(&mmu_context_idr, mm->context.id);
+	idr_remove(&mmu_context_idr, context_id);
 	spin_unlock(&mmu_context_lock);
+}
+EXPORT_SYMBOL_GPL(__destroy_context);
 
+void destroy_context(struct mm_struct *mm)
+{
+	__destroy_context(mm->context.id);
 	mm->context.id = NO_CONTEXT;
 }