arm64: head.S: 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.
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index b363f34..ffe9c2b6 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -415,14 +415,13 @@
*/
.set initial_sp, init_thread_union + THREAD_START_SP
__mmap_switched:
- adr_l x6, __bss_start
- adr_l x7, __bss_stop
+ // Clear BSS
+ adr_l x0, __bss_start
+ mov x1, xzr
+ adr_l x2, __bss_stop
+ sub x2, x2, x0
+ bl __pi_memset
-1: cmp x6, x7
- b.hs 2f
- str xzr, [x6], #8 // Clear BSS
- b 1b
-2:
adr_l sp, initial_sp, x4
mov x4, sp
and x4, x4, #~(THREAD_SIZE - 1)