Re: [RFC PATCH v4 2/5] target/riscv: rvv: Provide a fast path using direct access to host ram for unmasked unit-stride load/store

[Richard Henderson]

On 6/13/24 10:51, Max Chou wrote:
> This commit references the sve_ldN_r/sve_stN_r helper functions in ARM
> target to optimize the vector unmasked unit-stride load/store
> instructions by following items:
>
> * Get the loose bound of activate elements
> * Probing pages/resolving host memory address/handling watchpoint at beginning
> * Provide new interface to direct access host memory
>
> The original element load/store interface is replaced by the new element
> load/store functions with _tlb & _host postfix that means doing the
> element load/store through the original softmmu flow and the direct
> access host memory flow.
>
> Signed-off-by: Max Chou <max.chou@sifive.com>
> ---
>   target/riscv/insn_trans/trans_rvv.c.inc |   3 +
>   target/riscv/vector_helper.c            | 637 +++++++++++++++++++-----
>   target/riscv/vector_internals.h         |  48 ++
>   3 files changed, 551 insertions(+), 137 deletions(-)
>
> diff --git a/target/riscv/insn_trans/trans_rvv.c.inc 
> b/target/riscv/insn_trans/trans_rvv.c.inc
> index 3a3896ba06c..14e10568bd7 100644
> --- a/target/riscv/insn_trans/trans_rvv.c.inc
> +++ b/target/riscv/insn_trans/trans_rvv.c.inc
> @@ -770,6 +770,7 @@ static bool ld_us_mask_op(DisasContext *s, arg_vlm_v *a, 
> uint8_t eew)
>       /* Mask destination register are always tail-agnostic */
>       data = FIELD_DP32(data, VDATA, VTA, s->cfg_vta_all_1s);
>       data = FIELD_DP32(data, VDATA, VMA, s->vma);
> +    data = FIELD_DP32(data, VDATA, VM, 1);
>       return ldst_us_trans(a->rd, a->rs1, data, fn, s, false);
>   }
>   
> @@ -787,6 +788,7 @@ static bool st_us_mask_op(DisasContext *s, arg_vsm_v *a, uint8_t eew)
>       /* EMUL = 1, NFIELDS = 1 */
>       data = FIELD_DP32(data, VDATA, LMUL, 0);
>       data = FIELD_DP32(data, VDATA, NF, 1);
> +    data = FIELD_DP32(data, VDATA, VM, 1);
>       return ldst_us_trans(a->rd, a->rs1, data, fn, s, true);
>   }
>   
> @@ -1106,6 +1108,7 @@ static bool ldst_whole_trans(uint32_t vd, uint32_t rs1, uint32_t nf,
>       TCGv_i32 desc;
>   
>       uint32_t data = FIELD_DP32(0, VDATA, NF, nf);
> +    data = FIELD_DP32(data, VDATA, VM, 1);
>       dest = tcg_temp_new_ptr();
>       desc = tcg_constant_i32(simd_desc(s->cfg_ptr->vlenb,
>                                         s->cfg_ptr->vlenb, data));

This is ok, and would warrant a separate patch.


> +    if (vm == 0) {
> +        for (i = vstart; i < evl; ++i) {
> +            if (vext_elem_mask(v0, i)) {
> +                reg_idx_last = i;
> +                if (reg_idx_first < 0) {
> +                    reg_idx_first = i;
> +                }
> +            }
> +        }

This isn't great, and isn't used for now, since only unmasked unit-stride is handled so 
far.  I think this first patch should be simpler and *assume* VM is set.

> +/*
> + * Resolve the guest virtual addresses to info->page[].
> + * Control the generation of page faults with @fault.  Return false if
> + * there is no work to do, which can only happen with @fault == FAULT_NO.
> + */
> +static bool vext_cont_ldst_pages(CPURISCVState *env, RVVContLdSt *info,
> +                                 target_ulong addr, bool is_load,
> +                                 uint32_t desc, uint32_t esz, uintptr_t ra,
> +                                 bool is_us_whole)
> +{
> +    uint32_t vm = vext_vm(desc);
> +    uint32_t nf = vext_nf(desc);
> +    bool nofault = (vm == 1 ? false : true);

Why is nofault == "!vm"?

Also, it's silly to use ?: with true/false -- use the proper boolean expression in the 
first place.

That said... faults with RVV must interact with vstart.

I'm not sure what the best code organization is.

Perhaps a subroutine, passed the first and last elements for a single page.

  Update vstart, resolve the page, allowing the exception.
  If watchpoints, one call to cpu_check_watchpoint for the entire memory range.
  If ram, iterate through the rest of the page using host accesses; otherwise,
  iterate through the rest of the page using tlb accesses, making sure vstart
  is always up-to-date.

The main routine looks for the page_split, invokes the subroutine for the first (and 
likely only) page.  Special case any split-page element.  Invoke the subroutine for the 
second page.


r~