Re: [Qemu-devel] [PATCH] target/arm: Set strict alignment for ARMv6-M load/store

[Peter Maydell]

On 19 June 2018 at 21:42, Julia Suvorova <address@hidden> wrote:
> Unlike ARMv7-M, ARMv6-M only supports naturally aligned memory accesses
> for 16-bit halfword and 32-bit word accesses using the LDR, LDRH,
> LDRSH, STR and STRH instructions.
>
> Signed-off-by: Julia Suvorova <address@hidden>
> ---
>  target/arm/translate.c | 18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/target/arm/translate.c b/target/arm/translate.c
> index b988d379e7..d923cbe98e 100644
> --- a/target/arm/translate.c
> +++ b/target/arm/translate.c
> @@ -1100,7 +1100,14 @@ static inline TCGv gen_aa32_addr(DisasContext *s, 
> TCGv_i32 a32, TCGMemOp op)
>  static void gen_aa32_ld_i32(DisasContext *s, TCGv_i32 val, TCGv_i32 a32,
>                              int index, TCGMemOp opc)
>  {
> -    TCGv addr = gen_aa32_addr(s, a32, opc);
> +    TCGv addr;
> +
> +    if (arm_dc_feature(s, ARM_FEATURE_M) &&
> +        !arm_dc_feature(s, ARM_FEATURE_V7)) {
> +        opc |= MO_ALIGN;
> +    }

Hi; I think this is a good point to introduce a
ARM_FEATURE_M_MAIN feature bit, because this is one
of those places where v8M baseline and v6M are the same.
Basically:
 * add a line to the enum arm_features:
    ARM_FEATURE_M_MAIN, /* M profile Main Extension */
 * add set_feature() calls to cortex_m3/m4/m33_initfn()
   which set that feature
 * don't set the feature for the cortex-m0
 * in these checks, use
      if (arm_dc_feature(s, ARM_FEATURE_M) &&
          !arm_dc_feature(s, ARM_FEATURE_M_MAIN)) {

A lot of the v6M checks are going to also apply for v8M
baseline, so it'll be time saved later to use the feature
bit.

(Alignment checks are a bit more complicated as strictly
v7M has a config register bit to allow turning them on,
but let's not worry about that. We can refactor the code
later if we ever care about implementing that.)

thanks
-- PMM