[PATCH 3/6] x86-64: Put vsyscall_gtod_data at a fixed virtual address

[Andy Lutomirski]

This generates slightly better code because vclock_gettime doesn't
need to load the address.

Signed-off-by: Andy Lutomirski <luto@xxxxxxx>
---
 arch/x86/kernel/vmlinux.lds.S |   13 +++++++------
 arch/x86/vdso/vextern.h       |    9 ++++++++-
 2 files changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index bf47007..1f545ff 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -174,12 +174,6 @@ SECTIONS
 		*(.vsyscall_fn)
 	}
 
-	. = ALIGN(L1_CACHE_BYTES);
-	.vsyscall_gtod_data : AT(VLOAD(.vsyscall_gtod_data)) {
-		*(.vsyscall_gtod_data)
-	}
-
-	vsyscall_gtod_data = VVIRT(.vsyscall_gtod_data);
 	.vsyscall_clock : AT(VLOAD(.vsyscall_clock)) {
 		*(.vsyscall_clock)
 	}
@@ -208,6 +202,13 @@ SECTIONS
 		*(.vsyscall_3)
 	}
 
+	 /* This address must be kept in sync with arch/x86/vdso/vextern.h */
+	.vsyscall_gtod_data ADDR(.vsyscall_0) + 3072 + 256:
+	                    AT(VLOAD(.vsyscall_gtod_data)) {
+		*(.vsyscall_gtod_data)
+	}
+	vsyscall_gtod_data = VVIRT(.vsyscall_gtod_data);
+
 	. = __vsyscall_0 + PAGE_SIZE;
 
 #undef VSYSCALL_ADDR
diff --git a/arch/x86/vdso/vextern.h b/arch/x86/vdso/vextern.h
index 1683ba2..5bfa77c 100644
--- a/arch/x86/vdso/vextern.h
+++ b/arch/x86/vdso/vextern.h
@@ -2,6 +2,11 @@
 #include <asm/vsyscall.h>
 #define VEXTERN(x) \
 	extern typeof(x) *vdso_ ## x __attribute__((visibility("hidden")));
+#define VEXTERN_FIXED(x, offset) \
+	static typeof(x) * const vdso_ ## x = \
+		(void *)(VSYSCALL_START + (offset));
+#else
+#define VEXTERN_FIXED(x, offset)
 #endif
 
 #define VMAGIC 0xfeedbabeabcdefabUL
@@ -13,4 +18,6 @@
 
 VEXTERN(jiffies)
 VEXTERN(vgetcpu_mode)
-VEXTERN(vsyscall_gtod_data)
+VEXTERN_FIXED(vsyscall_gtod_data, 3072 + 256)  /* Hard-coded in linker script */
+
+#undef VEXTERN_FIXED
-- 
1.7.4

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