[PATCH 2/2] arm64: use memset to clear BSS

Currently we use an open-coded memzero to clear the BSS. As it is a
trivial implementation, it is sub-optimal.

Our optimised memset doesn't use the stack, is position-independent, and
for the memzero case can use of DC ZVA to clear large blocks
efficiently. In __mmap_switched the MMU is on and there are no live
caller-saved registers, so we can safely call an uninstrumented memset.

This patch changes __mmap_switched to use memset when clearing the BSS.
We use the __pi_memset alias so as to avoid any instrumentation in all
kernel configurations. As with the head symbols, we must get the linker
to generate __bss_size, as there is no ELF relocation for the
subtraction of two symbols.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <redacted>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <redacted>
Cc: Will Deacon <redacted>
---
 arch/arm64/kernel/head.S  | 14 ++++++--------
 arch/arm64/kernel/image.h |  2 ++
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 23cfc08..247a97b 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S

@@ -415,14 +415,12 @@ ENDPROC(__create_page_tables)
  */
 	.set	initial_sp, init_thread_union + THREAD_START_SP
 __mmap_switched:
-	adr_l	x6, __bss_start
-	adr_l	x7, __bss_stop
-
-1:	cmp	x6, x7
-	b.hs	2f
-	str	xzr, [x6], #8			// Clear BSS
-	b	1b
-2:
+	// clear BSS
+	adr_l	x0, __bss_start
+	mov	x1, xzr
+	mov_l	x2, __bss_size
+	bl	__pi_memset
+
 	adr_l	sp, initial_sp, x4
 	str_l	x21, __fdt_pointer, x5		// Save FDT pointer
 	str_l	x24, memstart_addr, x6		// Save PHYS_OFFSET

diff --git a/arch/arm64/kernel/image.h b/arch/arm64/kernel/image.h
index bc2abb8..5fd76b5 100644
--- a/arch/arm64/kernel/image.h
+++ b/arch/arm64/kernel/image.h

@@ -95,4 +95,6 @@ __efistub__edata		= _edata;
 
 #endif
 
+__bss_size			= __bss_stop - __bss_start;
+
 #endif /* __ASM_IMAGE_H */

-- 
1.9.1