[PATCH 4/5] KVM: s390: Allow skeys to be enabled for the current process

[Christian Borntraeger <borntraeger@xxxxxxxxxx>]

From: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>

Introduce a new function s390_enable_skey(), which enables storage key
handling via setting the use_skey flag in the mmu context.

This function is only useful within the context of kvm.

Note that enabling storage keys will cause a one-time hickup when
walking the page table; however, it saves us special effort for cases
like clear reset while making it possible for us to be architecture
conform.

s390_enable_skey() takes the page table lock to prevent reseting
storage keys triggered from multiple vcpus.

Signed-off-by: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
 arch/s390/include/asm/pgtable.h |  1 +
 arch/s390/mm/pgtable.c          | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
index a82678e..1852389 100644
--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -1678,6 +1678,7 @@ static inline pte_t mk_swap_pte(unsigned long type, unsigned long offset)
 extern int vmem_add_mapping(unsigned long start, unsigned long size);
 extern int vmem_remove_mapping(unsigned long start, unsigned long size);
 extern int s390_enable_sie(void);
+extern void s390_enable_skey(void);
 
 /*
  * No page table caches to initialise
diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index 02a3575..2ed28bc 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -1369,6 +1369,29 @@ int s390_enable_sie(void)
 }
 EXPORT_SYMBOL_GPL(s390_enable_sie);
 
+/*
+ * Enable storage key handling from now on and initialize the storage
+ * keys with the default key.
+ */
+void s390_enable_skey(void)
+{
+	/*
+	 * To avoid races between multiple vcpus, ending in calling
+	 * page_table_reset twice or more,
+	 * the page_table_lock is taken for serialization.
+	 */
+	spin_lock(&current->mm->page_table_lock);
+	if (mm_use_skey(current->mm)) {
+		spin_unlock(&current->mm->page_table_lock);
+		return;
+	}
+
+	current->mm->context.use_skey = 1;
+	spin_unlock(&current->mm->page_table_lock);
+	page_table_reset_pgste(current->mm, 0, TASK_SIZE, true);
+}
+EXPORT_SYMBOL_GPL(s390_enable_skey);
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 int pmdp_clear_flush_young(struct vm_area_struct *vma, unsigned long address,
 			   pmd_t *pmdp)
-- 
1.8.4.2

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