Re: [RFC PATCH 1/2] RISC-V: Add support for Ztso

[Daniel Henrique Barboza]

On 11/13/23 06:56, Christoph Muellner wrote:
> From: Palmer Dabbelt <palmer@rivosinc.com>
>
> The Ztso extension is already ratified, this adds it as a CPU property
> and adds various fences throughout the port in order to allow TSO
> targets to function on weaker hosts.  We need no fences for AMOs as
> they're already SC, the placess we need barriers are described.

s/placess/places

> These fences are placed in the RISC-V backend rather than TCG as is
> planned for x86-on-arm64 because RISC-V allows heterogenous (and

s/heterogenous/heterogeneous

> likely soon dynamic) hart memory models.
>
> Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
> Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
> ---


Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>

>   target/riscv/cpu.c                      |  2 ++
>   target/riscv/cpu_cfg.h                  |  1 +
>   target/riscv/insn_trans/trans_rva.c.inc | 11 ++++++++---
>   target/riscv/insn_trans/trans_rvi.c.inc | 16 ++++++++++++++--
>   target/riscv/insn_trans/trans_rvv.c.inc | 20 ++++++++++++++++++++
>   target/riscv/translate.c                |  3 +++
>   6 files changed, 48 insertions(+), 5 deletions(-)
>
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index 83c7c0cf07..b446e553b1 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -118,6 +118,7 @@ const RISCVIsaExtData isa_edata_arr[] = {
>       ISA_EXT_DATA_ENTRY(zksed, PRIV_VERSION_1_12_0, ext_zksed),
>       ISA_EXT_DATA_ENTRY(zksh, PRIV_VERSION_1_12_0, ext_zksh),
>       ISA_EXT_DATA_ENTRY(zkt, PRIV_VERSION_1_12_0, ext_zkt),
> +    ISA_EXT_DATA_ENTRY(ztso, PRIV_VERSION_1_12_0, ext_ztso),
>       ISA_EXT_DATA_ENTRY(zvbb, PRIV_VERSION_1_12_0, ext_zvbb),
>       ISA_EXT_DATA_ENTRY(zvbc, PRIV_VERSION_1_12_0, ext_zvbc),
>       ISA_EXT_DATA_ENTRY(zve32f, PRIV_VERSION_1_10_0, ext_zve32f),
> @@ -1336,6 +1337,7 @@ const RISCVCPUMultiExtConfig riscv_cpu_extensions[] = {
>       MULTI_EXT_CFG_BOOL("zksed", ext_zksed, false),
>       MULTI_EXT_CFG_BOOL("zksh", ext_zksh, false),
>       MULTI_EXT_CFG_BOOL("zkt", ext_zkt, false),
> +    MULTI_EXT_CFG_BOOL("ztso", ext_ztso, false),
>   
>       MULTI_EXT_CFG_BOOL("zdinx", ext_zdinx, false),
>       MULTI_EXT_CFG_BOOL("zfinx", ext_zfinx, false),
> diff --git a/target/riscv/cpu_cfg.h b/target/riscv/cpu_cfg.h
> index f4605fb190..a0f951d9c1 100644
> --- a/target/riscv/cpu_cfg.h
> +++ b/target/riscv/cpu_cfg.h
> @@ -70,6 +70,7 @@ struct RISCVCPUConfig {
>       bool ext_zihintntl;
>       bool ext_zihintpause;
>       bool ext_zihpm;
> +    bool ext_ztso;
>       bool ext_smstateen;
>       bool ext_sstc;
>       bool ext_svadu;
> diff --git a/target/riscv/insn_trans/trans_rva.c.inc 
> b/target/riscv/insn_trans/trans_rva.c.inc
> index 5f194a447b..85c7e31f2a 100644
> --- a/target/riscv/insn_trans/trans_rva.c.inc
> +++ b/target/riscv/insn_trans/trans_rva.c.inc
> @@ -28,7 +28,11 @@ static bool gen_lr(DisasContext *ctx, arg_atomic *a, MemOp 
> mop)
>           tcg_gen_mb(TCG_MO_ALL | TCG_BAR_STRL);
>       }
>       tcg_gen_qemu_ld_tl(load_val, src1, ctx->mem_idx, mop);
> -    if (a->aq) {
> +    /*
> +     * TSO defines AMOs as acquire+release-RCsc, but does not define LR/SC as
> +     * AMOs.  Instead treat them like loads.
> +     */
> +    if (a->aq || ctx->ztso) {
>           tcg_gen_mb(TCG_MO_ALL | TCG_BAR_LDAQ);
>       }
>   
> @@ -64,9 +68,10 @@ static bool gen_sc(DisasContext *ctx, arg_atomic *a, MemOp mop)
>       gen_set_label(l1);
>       /*
>        * Address comparison failure.  However, we still need to
> -     * provide the memory barrier implied by AQ/RL.
> +     * provide the memory barrier implied by AQ/RL/TSO.
>        */
> -    tcg_gen_mb(TCG_MO_ALL + a->aq * TCG_BAR_LDAQ + a->rl * TCG_BAR_STRL);
> +    TCGBar bar_strl = (ctx->ztso || a->rl) ? TCG_BAR_STRL : 0;
> +    tcg_gen_mb(TCG_MO_ALL + a->aq * TCG_BAR_LDAQ + bar_strl);
>       gen_set_gpr(ctx, a->rd, tcg_constant_tl(1));
>   
>       gen_set_label(l2);
> diff --git a/target/riscv/insn_trans/trans_rvi.c.inc 
> b/target/riscv/insn_trans/trans_rvi.c.inc
> index faf6d65064..ad40d3e87f 100644
> --- a/target/riscv/insn_trans/trans_rvi.c.inc
> +++ b/target/riscv/insn_trans/trans_rvi.c.inc
> @@ -266,12 +266,20 @@ static bool gen_load_i128(DisasContext *ctx, arg_lb *a, 
> MemOp memop)
>   
>   static bool gen_load(DisasContext *ctx, arg_lb *a, MemOp memop)
>   {
> +    bool out;
> +
>       decode_save_opc(ctx);
>       if (get_xl(ctx) == MXL_RV128) {
> -        return gen_load_i128(ctx, a, memop);
> +        out = gen_load_i128(ctx, a, memop);
>       } else {
> -        return gen_load_tl(ctx, a, memop);
> +        out = gen_load_tl(ctx, a, memop);
> +    }
> +
> +    if (ctx->ztso) {
> +        tcg_gen_mb(TCG_MO_ALL | TCG_BAR_LDAQ);
>       }
> +
> +    return out;
>   }
>   
>   static bool trans_lb(DisasContext *ctx, arg_lb *a)
> @@ -328,6 +336,10 @@ static bool gen_store_tl(DisasContext *ctx, arg_sb *a, 
> MemOp memop)
>       TCGv addr = get_address(ctx, a->rs1, a->imm);
>       TCGv data = get_gpr(ctx, a->rs2, EXT_NONE);
>   
> +    if (ctx->ztso) {
> +        tcg_gen_mb(TCG_MO_ALL | TCG_BAR_STRL);
> +    }
> +
>       tcg_gen_qemu_st_tl(data, addr, ctx->mem_idx, memop);
>       return true;
>   }
> diff --git a/target/riscv/insn_trans/trans_rvv.c.inc 
> b/target/riscv/insn_trans/trans_rvv.c.inc
> index 78bd363310..76e63fcbca 100644
> --- a/target/riscv/insn_trans/trans_rvv.c.inc
> +++ b/target/riscv/insn_trans/trans_rvv.c.inc
> @@ -636,8 +636,28 @@ static bool ldst_us_trans(uint32_t vd, uint32_t rs1, 
> uint32_t data,
>       tcg_gen_addi_ptr(dest, tcg_env, vreg_ofs(s, vd));
>       tcg_gen_addi_ptr(mask, tcg_env, vreg_ofs(s, 0));
>   
> +    /*
> +     * According to the specification
> +     *
> +     *   Additionally, if the Ztso extension is implemented, then vector memory
> +     *   instructions in the V extension and Zve family of extensions follow
> +     *   RVTSO at the instruction level.  The Ztso extension does not
> +     *   strengthen the ordering of intra-instruction element accesses.
> +     *
> +     * as a result neither ordered nor unordered accesses from the V
> +     * instructions need ordering within the loop but we do still need barriers
> +     * around the loop.
> +     */
> +    if (is_store && s->ztso) {
> +        tcg_gen_mb(TCG_MO_ALL | TCG_BAR_STRL);
> +    }
> +
>       fn(dest, mask, base, tcg_env, desc);
>   
> +    if (!is_store && s->ztso) {
> +        tcg_gen_mb(TCG_MO_ALL | TCG_BAR_LDAQ);
> +    }
> +
>       if (!is_store) {
>           mark_vs_dirty(s);
>       }
> diff --git a/target/riscv/translate.c b/target/riscv/translate.c
> index f0be79bb16..ab56051d6d 100644
> --- a/target/riscv/translate.c
> +++ b/target/riscv/translate.c
> @@ -109,6 +109,8 @@ typedef struct DisasContext {
>       /* PointerMasking extension */
>       bool pm_mask_enabled;
>       bool pm_base_enabled;
> +    /* Ztso */
> +    bool ztso;
>       /* Use icount trigger for native debug */
>       bool itrigger;
>       /* FRM is known to contain a valid value. */
> @@ -1194,6 +1196,7 @@ static void riscv_tr_init_disas_context(DisasContextBase 
> *dcbase, CPUState *cs)
>       ctx->cs = cs;
>       ctx->pm_mask_enabled = FIELD_EX32(tb_flags, TB_FLAGS, PM_MASK_ENABLED);
>       ctx->pm_base_enabled = FIELD_EX32(tb_flags, TB_FLAGS, PM_BASE_ENABLED);
> +    ctx->ztso = cpu->cfg.ext_ztso;
>       ctx->itrigger = FIELD_EX32(tb_flags, TB_FLAGS, ITRIGGER);
>       ctx->zero = tcg_constant_tl(0);
>       ctx->virt_inst_excp = false;