Re: [Qemu-devel] [PATCH 4/4] target-arm: UNDEF in the UNPREDICTABLE SRS-from-System case

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 11.02.2016 22:11, Peter Maydell wrote:
> Make get_r13_banked() raise an exception at runtime for the
> corner case of SRS from System mode, so that we can UNDEF it;
> this brings us in to line with the ARM ARM's set of permitted
> CONSTRAINED UNPREDICTABLE choices.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

It's a bit misleading that the name "get_r13_banked" says nothing about
SRS instruction but raises an SRS-specific exception. Though, it's only
used for SRS and there seems to be no other candidate to use it; so

Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>

> ---
>  target-arm/op_helper.c | 8 ++++++++
>  target-arm/translate.c | 9 +++++----
>  2 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/target-arm/op_helper.c b/target-arm/op_helper.c
> index 05f97a7..8183108 100644
> --- a/target-arm/op_helper.c
> +++ b/target-arm/op_helper.c
> @@ -474,6 +474,14 @@ uint32_t HELPER(get_r13_banked)(CPUARMState *env, uint32_t mode)
>  #if defined(CONFIG_USER_ONLY)
>      g_assert_not_reached();
>  #endif
> +    if ((env->uncached_cpsr & CPSR_M) == ARM_CPU_MODE_SYS) {
> +        /* SRS instruction is UNPREDICTABLE from System mode; we UNDEF.
> +         * Other UNPREDICTABLE and UNDEF cases were caught at translate time.
> +         */
> +        raise_exception(env, EXCP_UDEF, syn_uncategorized(),
> +                        exception_target_el(env));
> +    }
> +
>      if ((env->uncached_cpsr & CPSR_M) == mode) {
>          return env->regs[13];
>      } else {
> diff --git a/target-arm/translate.c b/target-arm/translate.c
> index 7bceb05..e69145d 100644
> --- a/target-arm/translate.c
> +++ b/target-arm/translate.c
> @@ -7590,10 +7590,7 @@ static void gen_srs(DisasContext *s,
>       * -- not a valid mode number
>       * -- a mode that's at a higher exception level
>       * -- Monitor, if we are Non-secure
> -     * For the UNPREDICTABLE cases we choose to UNDEF, except that for
> -     * "current mode is System" we will write a garbage SPSR.
> -     * (This is because we don't have access to our current mode here
> -     * and would have to do a runtime check to UNDEF for System.)
> +     * For the UNPREDICTABLE cases we choose to UNDEF.
>       */
>      if (s->current_el == 1 && !s->ns) {
>          gen_exception_insn(s, 4, EXCP_UDEF, syn_uncategorized(), 3);
> @@ -7639,6 +7636,9 @@ static void gen_srs(DisasContext *s,
>  
>      addr = tcg_temp_new_i32();
>      tmp = tcg_const_i32(mode);
> +    /* get_r13_banked() will raise an exception if called from System mode */
> +    gen_set_condexec(s);
> +    gen_set_pc_im(s, s->pc - 4);
>      gen_helper_get_r13_banked(addr, cpu_env, tmp);
>      tcg_temp_free_i32(tmp);
>      switch (amode) {
> @@ -7688,6 +7688,7 @@ static void gen_srs(DisasContext *s,
>          tcg_temp_free_i32(tmp);
>      }
>      tcg_temp_free_i32(addr);
> +    s->is_jmp = DISAS_UPDATE;
>  }
>  
>  static void disas_arm_insn(DisasContext *s, unsigned int insn)