Re: [PATCH 3/4] target/arm: Take an exception if PC is misaligned

[Peter Maydell]

On Wed, 18 Aug 2021 at 02:04, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> For A64, any input to an indirect branch can cause this.
>
> For A32, many indirect branch paths force the branch to be aligned,
> but BXWritePC does not.  This includes the BX instruction but also
> other interworking changes to PC.  Prior to v8, this case is UNDEFINED.
> With v8, this is CONSTRAINED UNDEFINED and may either raise an
> exception or force align the PC.
>
> We choose to raise an exception because we have the infrastructure,
> it makes the generated code for gen_bx simpler, and it has the
> possibility of catching more guest bugs.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>  static void arm_post_translate_insn(DisasContext *dc)
>  {
>      if (dc->condjmp && !dc->base.is_jmp) {
> @@ -9500,7 +9504,25 @@ static void arm_tr_translate_insn(DisasContextBase 
> *dcbase, CPUState *cpu)
>      CPUARMState *env = cpu->env_ptr;
>      unsigned int insn;
>
> -    if (arm_pre_translate_insn(dc)) {
> +    /* Singlestep exceptions have the highest priority. */
> +    if (arm_check_ss_active(dc)) {
> +        dc->base.pc_next += 4;
> +        return;
> +    }
> +
> +    if (dc->base.pc_next & 3) {
> +        /*
> +         * PC alignment fault.  This has priority over the instruction abort
> +         * that we would receive from a translation fault via arm_ldl_code
> +         * (or the execution of the kernelpage entrypoint).
> +         */
> +        gen_exception_insn(dc, dc->base.pc_next, EXCP_UDEF,
> +                           syn_pcalignment(), default_exception_el(dc));
> +        dc->base.pc_next = QEMU_ALIGN_UP(dc->base.pc_next, 4);
> +        return;
> +    }
> +
> +    if (arm_check_kernelpage(dc)) {
>          dc->base.pc_next += 4;
>          return;
>      }
> @@ -9570,7 +9592,7 @@ static void thumb_tr_translate_insn(DisasContextBase 
> *dcbase, CPUState *cpu)
>      uint32_t insn;
>      bool is_16bit;
>
> -    if (arm_pre_translate_insn(dc)) {
> +    if (arm_check_ss_active(dc) || arm_check_kernelpage(dc)) {


Is it not possible to get a misaligned PC in the Thumb case ?

>          dc->base.pc_next += 2;
>          return;
>      }

-- PMM