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

[Puranjay Mohan]

Xu Kuohai <xukuohai at huaweicloud.com> writes:

> 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
>

Acked-by: Puranjay Mohan <puranjay at kernel.org>

Thanks,
Puranjay Mohan
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 255 bytes
Desc: not available
URL: <http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20240826/7e437355/attachment.sig>