[PATCH 2/2] arm64: entry: use ldp/stp instead of push/pop when saving/restoring regs
Will Deacon
will.deacon at arm.com
Fri Nov 7 05:32:22 PST 2014
The push/pop instructions can be suboptimal when saving/restoring large
amounts of data to/from the stack, for example on entry/exit from the
kernel. This is because:
(1) They act on descending addresses (i.e. the newly decremented sp),
which may defeat some hardware prefetchers
(2) They introduce an implicit dependency between each instruction, as
the sp has to be updated in order to resolve the address of the
next access.
This patch removes the push/pop instructions from our kernel entry/exit
macros in favour of ldp/stp plus offset.
Signed-off-by: Will Deacon <will.deacon at arm.com>
---
arch/arm64/kernel/entry.S | 75 +++++++++++++++++++++++------------------------
1 file changed, 37 insertions(+), 38 deletions(-)
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 2cebe56d650c..622a409916f3 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -64,25 +64,26 @@
#define BAD_ERROR 3
.macro kernel_entry, el, regsize = 64
- sub sp, sp, #S_FRAME_SIZE - S_LR // room for LR, SP, SPSR, ELR
+ sub sp, sp, #S_FRAME_SIZE
.if \regsize == 32
mov w0, w0 // zero upper 32 bits of x0
.endif
- push x28, x29
- push x26, x27
- push x24, x25
- push x22, x23
- push x20, x21
- push x18, x19
- push x16, x17
- push x14, x15
- push x12, x13
- push x10, x11
- push x8, x9
- push x6, x7
- push x4, x5
- push x2, x3
- push x0, x1
+ stp x0, x1, [sp, #16 * 0]
+ stp x2, x3, [sp, #16 * 1]
+ stp x4, x5, [sp, #16 * 2]
+ stp x6, x7, [sp, #16 * 3]
+ stp x8, x9, [sp, #16 * 4]
+ stp x10, x11, [sp, #16 * 5]
+ stp x12, x13, [sp, #16 * 6]
+ stp x14, x15, [sp, #16 * 7]
+ stp x16, x17, [sp, #16 * 8]
+ stp x18, x19, [sp, #16 * 9]
+ stp x20, x21, [sp, #16 * 10]
+ stp x22, x23, [sp, #16 * 11]
+ stp x24, x25, [sp, #16 * 12]
+ stp x26, x27, [sp, #16 * 13]
+ stp x28, x29, [sp, #16 * 14]
+
.if \el == 0
mrs x21, sp_el0
get_thread_info tsk // Ensure MDSCR_EL1.SS is clear,
@@ -118,33 +119,31 @@
.if \el == 0
ct_user_enter
ldr x23, [sp, #S_SP] // load return stack pointer
+ msr sp_el0, x23
.endif
+ msr elr_el1, x21 // set up the return data
+ msr spsr_el1, x22
.if \ret
ldr x1, [sp, #S_X1] // preserve x0 (syscall return)
- add sp, sp, S_X2
.else
- pop x0, x1
- .endif
- pop x2, x3 // load the rest of the registers
- pop x4, x5
- pop x6, x7
- pop x8, x9
- msr elr_el1, x21 // set up the return data
- msr spsr_el1, x22
- .if \el == 0
- msr sp_el0, x23
+ ldp x0, x1, [sp, #16 * 0]
.endif
- pop x10, x11
- pop x12, x13
- pop x14, x15
- pop x16, x17
- pop x18, x19
- pop x20, x21
- pop x22, x23
- pop x24, x25
- pop x26, x27
- pop x28, x29
- ldr lr, [sp], #S_FRAME_SIZE - S_LR // load LR and restore SP
+ ldp x2, x3, [sp, #16 * 1]
+ ldp x4, x5, [sp, #16 * 2]
+ ldp x6, x7, [sp, #16 * 3]
+ ldp x8, x9, [sp, #16 * 4]
+ ldp x10, x11, [sp, #16 * 5]
+ ldp x12, x13, [sp, #16 * 6]
+ ldp x14, x15, [sp, #16 * 7]
+ ldp x16, x17, [sp, #16 * 8]
+ ldp x18, x19, [sp, #16 * 9]
+ ldp x20, x21, [sp, #16 * 10]
+ ldp x22, x23, [sp, #16 * 11]
+ ldp x24, x25, [sp, #16 * 12]
+ ldp x26, x27, [sp, #16 * 13]
+ ldp x28, x29, [sp, #16 * 14]
+ ldr lr, [sp, #S_LR]
+ add sp, sp, #S_FRAME_SIZE // restore sp
eret // return to kernel
.endm
--
2.1.1
More information about the linux-arm-kernel
mailing list