[v5 09/15] sparc64: optimized struct page zeroing

[Pavel Tatashin]

Add an optimized mm_zero_struct_page(), so struct page's are zeroed without
calling memset(). We do eight regular stores, thus avoid cost of membar.

Signed-off-by: Pavel Tatashin <pasha.tatashin at oracle.com>
Reviewed-by: Steven Sistare <steven.sistare at oracle.com>
Reviewed-by: Daniel Jordan <daniel.m.jordan at oracle.com>
Reviewed-by: Bob Picco <bob.picco at oracle.com>
---
 arch/sparc/include/asm/pgtable_64.h | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/arch/sparc/include/asm/pgtable_64.h b/arch/sparc/include/asm/pgtable_64.h
index 6fbd931f0570..be47537e84c5 100644
--- a/arch/sparc/include/asm/pgtable_64.h
+++ b/arch/sparc/include/asm/pgtable_64.h
@@ -230,6 +230,38 @@ extern unsigned long _PAGE_ALL_SZ_BITS;
 extern struct page *mem_map_zero;
 #define ZERO_PAGE(vaddr)	(mem_map_zero)
 
+/* This macro must be updated when the size of struct page grows above 80
+ * or reduces below 64.
+ * The idea that compiler optimizes out switch() statement, and only
+ * leaves clrx instructions or memset() call.
+ */
+#define	mm_zero_struct_page(pp) do {					\
+	unsigned long *_pp = (void *)(pp);				\
+									\
+	/* Check that struct page is 8-byte aligned */			\
+	BUILD_BUG_ON(sizeof(struct page) & 7);				\
+									\
+	switch (sizeof(struct page)) {					\
+	case 80:							\
+		_pp[9] = 0;	/* fallthrough */			\
+	case 72:							\
+		_pp[8] = 0;	/* fallthrough */			\
+	case 64:							\
+		_pp[7] = 0;						\
+		_pp[6] = 0;						\
+		_pp[5] = 0;						\
+		_pp[4] = 0;						\
+		_pp[3] = 0;						\
+		_pp[2] = 0;						\
+		_pp[1] = 0;						\
+		_pp[0] = 0;						\
+		break;		/* no fallthrough */			\
+	default:							\
+		pr_warn_once("suboptimal mm_zero_struct_page");		\
+		memset(_pp, 0, sizeof(struct page));			\
+	}								\
+} while (0)
+
 /* PFNs are real physical page numbers.  However, mem_map only begins to record
  * per-page information starting at pfn_base.  This is to handle systems where
  * the first physical page in the machine is at some huge physical address,
-- 
2.13.4