[PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until the first LAM user

[Kirill A. Shutemov]

Use static key to reduce untagged_addr() overhead.

The key only gets enabled when the first process enables LAM.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/uaccess.h | 8 ++++++--
 arch/x86/kernel/process_64.c   | 4 ++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 32c9dd052e43..f9f85d596581 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -24,6 +24,8 @@ static inline bool pagefault_disabled(void);
 #endif
 
 #ifdef CONFIG_ADDRESS_MASKING
+DECLARE_STATIC_KEY_FALSE(tagged_addr_key);
+
 /*
  * Mask out tag bits from the address.
  *
@@ -32,8 +34,10 @@ static inline bool pagefault_disabled(void);
  */
 #define __untagged_addr(untag_mask, addr)	({			\
 	u64 __addr = (__force u64)(addr);				\
-	s64 sign = (s64)__addr >> 63;					\
-	__addr &= untag_mask | sign;					\
+	if (static_branch_likely(&tagged_addr_key)) {			\
+		s64 sign = (s64)__addr >> 63;				\
+		__addr &= untag_mask | sign;				\
+	}								\
 	(__force __typeof__(addr))__addr;				\
 })
 
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 88aae519c8f8..1b41c60ebf6e 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -745,6 +745,9 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
 
 #ifdef CONFIG_ADDRESS_MASKING
 
+DEFINE_STATIC_KEY_FALSE(tagged_addr_key);
+EXPORT_SYMBOL_GPL(tagged_addr_key);
+
 #define LAM_U57_BITS 6
 
 static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
@@ -781,6 +784,7 @@ static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
 
 	mmap_write_unlock(mm);
 
+	static_branch_enable(&tagged_addr_key);
 	return 0;
 }
 #endif
-- 
2.38.2