Re: [Qemu-devel] [PATCH for-2.8] target-arm/translate-a64: fix gen_load_exclusive

["Alex Bennée" <alex.bennee@linaro.org>]

Alex Bennée <alex.bennee@linaro.org> writes:

> While testing rth's latest TCG patches with risu I found ldaxp was
> broken. Investigating further I found it was broken by 1dd089d0 when
> the cmpxchg atomic work was merged.

CC'ing Paolo/Richard

Do you guys have any final TCG fixes planned for 2.8 that can take this
fix for the ldaxp regression?

> As part of that change the code
> attempted to be clever by doing a single 64 bit load and then shuffle
> the data around to set the two 32 bit registers.
>
> As I couldn't quite follow the endian magic I've simply partially
> reverted the change to the original code gen_load_exclusive code. This
> doesn't affect the cmpxchg functionality as that is all done on in
> gen_store_exclusive part which is untouched.
>
> I've also restored the comment that was removed (with a slight tweak
> to mention cmpxchg).
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
>  target-arm/translate-a64.c | 42 +++++++++++++++++++-----------------------
>  1 file changed, 19 insertions(+), 23 deletions(-)
>
> diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
> index de48747..6dc27a6 100644
> --- a/target-arm/translate-a64.c
> +++ b/target-arm/translate-a64.c
> @@ -1839,41 +1839,37 @@ static void disas_b_exc_sys(DisasContext *s, uint32_t insn)
>      }
>  }
>
> +/*
> + * Load/Store exclusive instructions are implemented by remembering
> + * the value/address loaded, and seeing if these are the same
> + * when the store is performed. This is not actually the architecturally
> + * mandated semantics, but it works for typical guest code sequences
> + * and avoids having to monitor regular stores.
> + *
> + * The store exclusive uses the atomic cmpxchg primitives to avoid
> + * races in multi-threaded linux-user and when MTTCG softmmu is
> + * enabled.
> + */
>  static void gen_load_exclusive(DisasContext *s, int rt, int rt2,
>                                 TCGv_i64 addr, int size, bool is_pair)
>  {
>      TCGv_i64 tmp = tcg_temp_new_i64();
> -    TCGMemOp be = s->be_data;
> +    TCGMemOp memop = s->be_data + size;
>
>      g_assert(size <= 3);
> +    tcg_gen_qemu_ld_i64(tmp, addr, get_mem_index(s), memop);
> +
>      if (is_pair) {
> +        TCGv_i64 addr2 = tcg_temp_new_i64();
>          TCGv_i64 hitmp = tcg_temp_new_i64();
>
> -        if (size == 3) {
> -            TCGv_i64 addr2 = tcg_temp_new_i64();
> -
> -            tcg_gen_qemu_ld_i64(tmp, addr, get_mem_index(s),
> -                                MO_64 | MO_ALIGN_16 | be);
> -            tcg_gen_addi_i64(addr2, addr, 8);
> -            tcg_gen_qemu_ld_i64(hitmp, addr2, get_mem_index(s),
> -                                MO_64 | MO_ALIGN | be);
> -            tcg_temp_free_i64(addr2);
> -        } else {
> -            g_assert(size == 2);
> -            tcg_gen_qemu_ld_i64(tmp, addr, get_mem_index(s),
> -                                MO_64 | MO_ALIGN | be);
> -            if (be == MO_LE) {
> -                tcg_gen_extr32_i64(tmp, hitmp, tmp);
> -            } else {
> -                tcg_gen_extr32_i64(hitmp, tmp, tmp);
> -            }
> -        }
> -
> +        g_assert(size >= 2);
> +        tcg_gen_addi_i64(addr2, addr, 1 << size);
> +        tcg_gen_qemu_ld_i64(hitmp, addr2, get_mem_index(s), memop);
> +        tcg_temp_free_i64(addr2);
>          tcg_gen_mov_i64(cpu_exclusive_high, hitmp);
>          tcg_gen_mov_i64(cpu_reg(s, rt2), hitmp);
>          tcg_temp_free_i64(hitmp);
> -    } else {
> -        tcg_gen_qemu_ld_i64(tmp, addr, get_mem_index(s), size | MO_ALIGN | be);
>      }
>
>      tcg_gen_mov_i64(cpu_exclusive_val, tmp);


--
Alex Bennée