[PATCH bpf-next 0/2] bpf, arm64: Simplify jited prologue/epilogue

[Xu Kuohai]

From: Xu Kuohai <xukuohai at huawei.com>

The arm64 jit blindly saves/restores all callee-saved registers, making
the jited result looks a bit too compliated. For example, for an empty
prog, the jited result is:

   0:   bti jc
   4:   mov     x9, lr
   8:   nop
   c:   paciasp
  10:   stp     fp, lr, [sp, #-16]!
  14:   mov     fp, sp
  18:   stp     x19, x20, [sp, #-16]!
  1c:   stp     x21, x22, [sp, #-16]!
  20:   stp     x26, x25, [sp, #-16]!
  24:   mov     x26, #0
  28:   stp     x26, x25, [sp, #-16]!
  2c:   mov     x26, sp
  30:   stp     x27, x28, [sp, #-16]!
  34:   mov     x25, sp
  38:   bti j 		// tailcall target
  3c:   sub     sp, sp, #0
  40:   mov     x7, #0
  44:   add     sp, sp, #0
  48:   ldp     x27, x28, [sp], #16
  4c:   ldp     x26, x25, [sp], #16
  50:   ldp     x26, x25, [sp], #16
  54:   ldp     x21, x22, [sp], #16
  58:   ldp     x19, x20, [sp], #16
  5c:   ldp     fp, lr, [sp], #16
  60:   mov     x0, x7
  64:   autiasp
  68:   ret

Clearly, there is no need to save/restore unused callee-saved registers.
This patch does this change, making the jited image to only save/restore
the callee-saved registers it uses.

Now the jited result of empty prog is:

   0:   bti jc
   4:   mov     x9, lr
   8:   nop
   c:   paciasp
  10:   stp     fp, lr, [sp, #-16]!
  14:   mov     fp, sp
  18:   stp     xzr, x26, [sp, #-16]!
  1c:   mov     x26, sp
  20:   bti j		// tailcall target
  24:   mov     x7, #0
  28:   ldp     xzr, x26, [sp], #16
  2c:   ldp     fp, lr, [sp], #16
  30:   mov     x0, x7
  34:   autiasp
  38:   ret

Xu Kuohai (2):
  bpf, arm64: Get rid of fpb
  bpf, arm64: Avoid blindly saving/restoring all callee-saved registers

 arch/arm64/net/bpf_jit_comp.c | 394 +++++++++++++++++-----------------
 1 file changed, 192 insertions(+), 202 deletions(-)

-- 
2.43.0