RISC-V: Make BSS section as the last section in vmlinux.lds.S

avpatel · palmer-dabbelt · commit 22e6a2e14cb8 · 2019-01-07T07:59:25.000-08:00
The objcopy only emits loadable sections when creating flat kernel
Image. To have minimal possible size of flat kernel Image, we should
have all non-loadable sections after loadable sections.

Currently, execption table section (loadable section) is after BSS
section (non-loadable section) in the RISC-V vmlinux.lds.S. This
is not optimal for having minimal flat kernel Image size hence this
patch makes BSS section as the last section in RISC-V vmlinux.lds.S.

In addition, we make BSS section aligned to 16byte instead of PAGE
aligned which further reduces flat kernel Image size by few KBs.

The flat kernel Image size of Linux-4.20-rc4 using GCC 8.2.0 is
8819980 bytes with current RISC-V vmlinux.lds.S and it reduces to
7991740 bytes with this patch applied. In summary, this patch reduces
Linux-4.20-rc4 flat kernel Image size by 809 KB.

Signed-off-by: Anup Patel &lt;anup@brainfault.org&gt;
Reviewed-by: Bin Meng &lt;bmeng.cn@gmail.com&gt;
Signed-off-by: Palmer Dabbelt &lt;palmer@sifive.com&gt;
diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
@@ -18,6 +18,8 @@
 #include <asm/cache.h>
 #include <asm/thread_info.h>
 
+#define MAX_BYTES_PER_LONG	0x10
+
 OUTPUT_ARCH(riscv)
 ENTRY(_start)
 
@@ -74,15 +76,17 @@ SECTIONS
 		*(.sbss*)
 	}
 
-	BSS_SECTION(PAGE_SIZE, PAGE_SIZE, 0)
-
 	EXCEPTION_TABLE(0x10)
 	NOTES
 
 	.rel.dyn : {
 		*(.rel.dyn*)
 	}
 
+	BSS_SECTION(MAX_BYTES_PER_LONG,
+		    MAX_BYTES_PER_LONG,
+		    MAX_BYTES_PER_LONG)
+
 	_end = .;
 
 	STABS_DEBUG
