Re: [PATCH v2 7/7] tcg/ppc: Use prefixed instructions for tcg_out_goto_tb

[Jordan Niethe]

On Tue, Aug 8, 2023 at 1:02 PM Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> When a direct branch is out of range, we can load the destination for
> the indirect branch using PLA (for 16GB worth of buffer) and PLD from
> the TranslationBlock for everything larger.
>
> This means the patch affects exactly one instruction: B (plus filler),
> PLA or PLD.  Which means we can update and execute the patch atomically.

I think the commit message needs to be updated for Nick's changes.

>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  tcg/ppc/tcg-target.c.inc | 31 +++++++++++++++++++------------
>  1 file changed, 19 insertions(+), 12 deletions(-)
>
> diff --git a/tcg/ppc/tcg-target.c.inc b/tcg/ppc/tcg-target.c.inc
> index 63fe4ef995..b686a68247 100644
> --- a/tcg/ppc/tcg-target.c.inc
> +++ b/tcg/ppc/tcg-target.c.inc
> @@ -2646,31 +2646,38 @@ static void tcg_out_goto_tb(TCGContext *s, int which)
>      uintptr_t ptr = get_jmp_target_addr(s, which);
>
>      if (USE_REG_TB) {
> +        /*
> +         * With REG_TB, we must always use indirect branching,
> +         * so that the branch destination and TCG_REG_TB match.
> +         */
>          ptrdiff_t offset = tcg_tbrel_diff(s, (void *)ptr);
>          tcg_out_mem_long(s, LD, LDX, TCG_REG_TB, TCG_REG_TB, offset);
> -
> -        /* TODO: Use direct branches when possible. */
> -        set_jmp_insn_offset(s, which);
>          tcg_out32(s, MTSPR | RS(TCG_REG_TB) | CTR);
> -
>          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);
> +        return;
> +    }
>
> -        /* When branch is out of range, fall through to indirect. */
> +    /* 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. */
> +    if (have_isa_3_10) {
> +        ptrdiff_t offset = tcg_pcrel_diff_for_prefix(s, (void *)ptr);
> +        tcg_out_8ls_d(s, PLD, TCG_REG_TMP1, 0, offset, 1);
> +    } else {
>          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);
>      }
> +
> +    tcg_out32(s, MTSPR | RS(TCG_REG_TMP1) | CTR);
> +    tcg_out32(s, BCCTR | BO_ALWAYS);
> +    set_jmp_reset_offset(s, which);
>  }
>
>  void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
> --
> 2.34.1
>

Thank you for implementing this Richard.

I was able to boot mttcg guests on P9 and P10 hosts.

Tested-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Jordan Niethe <jniethe5@gmail.com>