Re: [Xen-devel] [Very RFC PATCH 2/3] insn: Add arch64_insn_gen_branch_imm to generate branch

[Julien Grall <julien.grall@xxxxxxx>]

Hi Konrad,

On 09/08/2016 06:18, Konrad Rzeszutek Wilk wrote:
> We copied from Linux code (v4.7) the code that generates
> the unconditional branch instructions. This is mostly
> from Linux commit c0cafbae20d2878883ec3c06d6ea30ff38a6bf92
> "arm64: introduce aarch64_insn_gen_branch_reg()"
>
> However the branch-link instructions was omitted, only
> the branch instruction is generated.
I would prefer to have the function fully copied (i.e with the 
branch-link instructions). This will make easier to re-sync the code 
later on.
> This lays the groundwork for Livepatch to generate the
> trampoline to jump to the new replacement function.
>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> --
> Cc: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>
> ---
>  xen/arch/arm/arm64/insn.c        | 41 ++++++++++++++++++++++++++++++++++++++++
>  xen/include/asm-arm/arm64/insn.h |  2 ++
>  2 files changed, 43 insertions(+)
>
> diff --git a/xen/arch/arm/arm64/insn.c b/xen/arch/arm/arm64/insn.c
> index 12b4d96..15dd7bc 100644
> --- a/xen/arch/arm/arm64/insn.c
> +++ b/xen/arch/arm/arm64/insn.c
> @@ -209,6 +209,47 @@ u32 aarch64_set_branch_offset(u32 insn, s32 offset)
>         BUG();
>  }
>
> +static inline long branch_imm_common(unsigned long pc, unsigned long addr,
> +                                    long range)
> +{
> +       long offset;
> +
> +       if ((pc & 0x3) || (addr & 0x3)) {
> +               pr_err("%s: A64 instructions must be word aligned\n", __func__);
> +               return range;
> +       }
> +
> +       offset = ((long)addr - (long)pc);
> +
> +       if (offset < -range || offset >= range) {
> +               pr_err("%s: offset out of range\n", __func__);
> +               return range;
> +       }
> +
> +       return offset;
> +}
> +
> +u32 __kprobes aarch64_insn_gen_branch_imm(unsigned long pc, unsigned long addr)
> +{
> +       u32 insn;
> +       long offset;
> +
> +       /*
> +        * B/BL support [-128M, 128M) offset
> +        * ARM64 virtual address arrangement guarantees all kernel and module
> +        * texts are within +/-128M.
> +        */
> +       offset = branch_imm_common(pc, addr, SZ_128M);
> +       if (offset >= SZ_128M)
> +               return AARCH64_BREAK_FAULT;
> +
> +       insn = aarch64_insn_get_b_value();
> +
> +       return aarch64_insn_encode_immediate(AARCH64_INSN_IMM_26, insn,
> +                                            offset >> 2);
> +}
I would prefer to have the functions added at the same place as in Linux 
for similar reason as stated previously.
> +
> +
I think you can drop one empty line here.

>  /*
>   * Local variables:
>   * mode: C
> diff --git a/xen/include/asm-arm/arm64/insn.h b/xen/include/asm-arm/arm64/insn.h
> index 6ce37be..378ba11 100644
> --- a/xen/include/asm-arm/arm64/insn.h
> +++ b/xen/include/asm-arm/arm64/insn.h
> @@ -61,6 +61,8 @@ u32 aarch64_insn_encode_immediate(enum aarch64_insn_imm_type 
> type,
>  s32 aarch64_get_branch_offset(u32 insn);
>  u32 aarch64_set_branch_offset(u32 insn, s32 offset);
>
> +u32 aarch64_insn_gen_branch_imm(unsigned long pc, unsigned long addr);
> +
>  /* Wrapper for common code */
>  static inline bool insn_is_branch_imm(u32 insn)
>  {
Regards,

--
Julien Grall

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