Re: [Qemu-devel] [PATCH v3 2/3] tcg/aarch64: Use ADRP+ADD to compute target address

[Richard Henderson]

On 06/29/2017 05:40 PM, Pranith Kumar wrote:
>   void aarch64_tb_set_jmp_target(uintptr_t jmp_addr, uintptr_t addr)
>   {
>       tcg_insn_unit *code_ptr = (tcg_insn_unit *)jmp_addr;
> -    tcg_insn_unit *target = (tcg_insn_unit *)addr;
> +    tcg_insn_unit i1, i2;
> +    uint64_t pair;
>   
> +    ptrdiff_t offset = addr - jmp_addr;
> +
> +    if (offset == sextract64(offset, 0, 26)) {
> +        i1 = NOP;
> +        i2 = I3206_B | ((offset >> 2) & 0x3ffffff);

Branch first, since that's the offset you calculated.
Also, the nop need not be executed.

> +    } else {
> +        offset = (addr >> 12) - (jmp_addr >> 12);
> +
> +        /* patch ADRP */
> +        i2 = deposit32(*code_ptr++, 29, 2, offset & 0x3);
> +        i2 = deposit32(i2, 5, 19, offset >> 2);
> +        /* patch ADDI */
> +        i1 = deposit32(*code_ptr, 10, 12, addr & 0xfff);

You can't just patch these insns, because they aren't necessarily ADRP+ADD. 
Indeed, they will very likely be B and NOP.  The first address we patch in is 
tb_jmp_reset_offset, which is the following opcode, which is definitely in 
range of the branch above.


r~