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

On Wed, Jan 06, 2016 at 12:12:45PM +0100, Ard Biesheuvel wrote:

On 6 January 2016 at 12:05, Mark Rutland [off-list ref] wrote:

quoted

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

Is it such a big deal to do

adr_l x2, __bss_stop
sub x2, x2, x0

instead?

I'm happy either way.

It no-one else has a use for mov_l I'll drop it and move to that.

Either way:
Reviewed-by: Ard Biesheuvel <redacted>

Thanks!

Mark.