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..ffe9c2b 100644 (file)
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -415,14 +415,13 @@ 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
+       adr_l   x2, __bss_stop
+       sub     x2, x2, x0
+       bl      __pi_memset
+
        adr_l   sp, initial_sp, x4
        mov     x4, sp
        and     x4, x4, #~(THREAD_SIZE - 1)