Re: [PATCH v2 17/22] tcg/ppc: Reorg goto_tb implementation

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

Richard Henderson <richard.henderson@linaro.org> writes:

> The old ppc64 implementation replaces 2 or 4 insns, which leaves a race
> condition in which a thread could be stopped at a PC in the middle of
> the sequence, and when restarted does not see the complete address
> computation and branches to nowhere.
>
> The new implemetation replaces only one insn, swapping between
>
> 	b       <dest>
> and
> 	mtctr	r31
>
> falling through to a general-case indirect branch.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
<snip>
>  
>  static void tcg_out_goto_tb(TCGContext *s, int which)
>  {
> -    /* Direct jump. */
> -    if (TCG_TARGET_REG_BITS == 64) {
> -        /* Ensure the next insns are 8 or 16-byte aligned. */
> -        while ((uintptr_t)s->code_ptr & (have_isa_2_07 ? 15 : 7)) {
> -            tcg_out32(s, NOP);
> -        }
> +    uintptr_t ptr = get_jmp_target_addr(s, which);
> +
> +    if (USE_REG_TB) {
> +        ptrdiff_t offset = tcg_tbrel_diff(s, (void *)ptr);
> +        tcg_out_mem_long(s, LD, LDX, TCG_REG_TB, TCG_REG_TB, offset);
> +    
> +        /* Direct branch will be patched by tb_target_set_jmp_target. */
>          set_jmp_insn_offset(s, which);
> -        tcg_out32(s, ADDIS | TAI(TCG_REG_TB, TCG_REG_TB, 0));
> -        tcg_out32(s, ADDI | TAI(TCG_REG_TB, TCG_REG_TB, 0));
>          tcg_out32(s, MTSPR | RS(TCG_REG_TB) | CTR);
> +
> +        /* When branch is out of range, fall through to indirect. */
> +        tcg_out32(s, BCCTR | BO_ALWAYS);
> +
> +        /* For the unlinked case, need to reset TCG_REG_TB.  */
> +        set_jmp_reset_offset(s, which);
> +        tcg_out_mem_long(s, ADDI, ADD, TCG_REG_TB, TCG_REG_TB,
> +                         -tcg_current_code_size(s));
> +    } else {
> +        /* Direct branch will be patched by tb_target_set_jmp_target. */
> +        set_jmp_insn_offset(s, which);
> +        tcg_out32(s, NOP);
> +
> +        /* When branch is out of range, fall through to indirect. */
> +        tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_TMP1, ptr - (int16_t)ptr);
> +        tcg_out_ld(s, TCG_TYPE_PTR, TCG_REG_TMP1, TCG_REG_TMP1, (int16_t)ptr);
> +        tcg_out32(s, MTSPR | RS(TCG_REG_TMP1) | CTR);
>          tcg_out32(s, BCCTR | BO_ALWAYS);
>          set_jmp_reset_offset(s, which);
> -        if (USE_REG_TB) {
> -            /* For the unlinked case, need to reset TCG_REG_TB.  */
> -            tcg_out_mem_long(s, ADDI, ADD, TCG_REG_TB, TCG_REG_TB,
> -                             -tcg_current_code_size(s));
> -        }
> -    } else {
> -        set_jmp_insn_offset(s, which);
> -        tcg_out32(s, B);
> -        set_jmp_reset_offset(s, which);
>      }
>  }
>  
> +void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
> +                              uintptr_t jmp_rx, uintptr_t jmp_rw)
> +{
> +    uintptr_t addr = tb->jmp_target_addr[n];
> +    intptr_t diff = addr - jmp_rx;
> +    tcg_insn_unit insn;
> +
> +    if (in_range_b(diff)) {
> +        insn = B | (diff & 0x3fffffc);

Again deposit would be nice here.

> +    } else if (USE_REG_TB) {
> +        insn = MTSPR | RS(TCG_REG_TB) | CTR;
> +    } else {
> +        insn = NOP;
> +    }
> +
> +    qatomic_set((uint32_t *)jmp_rw, insn);
> +    flush_idcache_range(jmp_rx, jmp_rw, 4);
> +}
> +
>  static void tcg_out_op(TCGContext *s, TCGOpcode opc,
>                         const TCGArg args[TCG_MAX_OP_ARGS],
>                         const int const_args[TCG_MAX_OP_ARGS])

Otherwise:

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro