Re: [PATCH v4 1/5] target/riscv: add vector unit stride load and store instructions

[LIU Zhiwei]

On 2020/2/28 3:17, Richard Henderson wrote:

> On 2/25/20 2:35 AM, LIU Zhiwei wrote:
> > +static bool vext_check_reg(DisasContext *s, uint32_t reg, bool widen)
> > +{
> > +    int legal = widen ? 2 << s->lmul : 1 << s->lmul;
> > +
> > +    return !((s->lmul == 0x3 && widen) || (reg % legal));
> > +}
> > +
> > +static bool vext_check_overlap_mask(DisasContext *s, uint32_t vd, bool vm)
> > +{
> > +    return !(s->lmul > 1 && vm == 0 && vd == 0);
> > +}
> > +
> > +static bool vext_check_nf(DisasContext *s, uint32_t nf)
> > +{
> > +    return s->lmul * (nf + 1) <= 8;
> > +}
> >       
> Some commentary would be good here, quoting the rule being applied.  E.g. "The
> destination vector register group for a masked vector instruction can only
> overlap the source mask regis-
> ter (v0) when LMUL=1. (Section 5.3)"
>     
    Good idea.
> > +static bool ld_us_op(DisasContext *s, arg_r2nfvm *a, uint8_t seq)
> > +{
> > +    uint8_t nf = a->nf + 1;
> >       
> Perhaps NF should have the +1 done during decode, so that it cannot be
> forgotten here or elsewhere.
>     
    
    Perhaps not. It will  not be used elsewhere. And it will need one
    more bit in FIELD().
>  E.g.
>
> %nf      31:3  !function=ex_plus_1
> @r2_nfvm ... ... vm:1 ..... ..... ... ..... ....... \
>          &r2nfvm %nf %rs1 %rd
>
> Where ex_plus_1 is the obvious modification of ex_shift_1().
>     
> > +static inline uint32_t vext_nf(uint32_t desc)
> > +{
> > +    return (simd_data(desc) >> 11) & 0xf;
> > +}
> > +
> > +static inline uint32_t vext_mlen(uint32_t desc)
> > +{
> > +    return simd_data(desc) & 0xff;
> > +}
> > +
> > +static inline uint32_t vext_vm(uint32_t desc)
> > +{
> > +    return (simd_data(desc) >> 8) & 0x1;
> > +}
> > +
> > +static inline uint32_t vext_lmul(uint32_t desc)
> > +{
> > +    return (simd_data(desc) >> 9) & 0x3;
> > +}
> >       
> You should use FIELD() to define the fields, and then use FIELD_EX32 and
> FIELD_DP32 to reference them.
>     
    Nice, I will find some place to define the fields.
> > +/*
> > + * This function checks watchpoint before real load operation.
> > + *
> > + * In softmmu mode, the TLB API probe_access is enough for watchpoint check.
> > + * In user mode, there is no watchpoint support now.
> > + *
> > + * It will triggle an exception if there is no mapping in TLB
> >       
> trigger.
>     
    Yes.
> > + * and page table walk can't fill the TLB entry. Then the guest
> > + * software can return here after process the exception or never return.
> > + */
> > +static void probe_read_access(CPURISCVState *env, target_ulong addr,
> > +        target_ulong len, uintptr_t ra)
> > +{
> > +    while (len) {
> > +        const target_ulong pagelen = -(addr | TARGET_PAGE_MASK);
> > +        const target_ulong curlen = MIN(pagelen, len);
> > +
> > +        probe_read(env, addr, curlen, cpu_mmu_index(env, false), ra);
> > +        addr += curlen;
> > +        len -= curlen;
> > +    }
> > +}
> > +
> > +static void probe_write_access(CPURISCVState *env, target_ulong addr,
> > +        target_ulong len, uintptr_t ra)
> > +{
> > +    while (len) {
> > +        const target_ulong pagelen = -(addr | TARGET_PAGE_MASK);
> > +        const target_ulong curlen = MIN(pagelen, len);
> > +
> > +        probe_write(env, addr, curlen, cpu_mmu_index(env, false), ra);
> > +        addr += curlen;
> > +        len -= curlen;
> > +    }
> > +}
> >       
> A loop is overkill -- the access cannot span to 3 pages.
>     
    Yes, I will just do as you suggest!
    
    In the unit stride load, without mask,  the max access len is
    checked . It is 512 in bytes.
    And current target page is 4096 in bytes.
    
    #define TARGET_PAGE_BITS 12 /* 4 KiB Pages */
    
> These two functions
> can be merged using probe_access and MMU_DATA_{LOAD,STORE}.
>
> > +
> > +#ifdef HOST_WORDS_BIGENDIAN
> > +static void vext_clear(void *tail, uint32_t cnt, uint32_t tot)
> > +{
> > +    /*
> > +     * Split the remaining range to two parts.
> > +     * The first part is in the last uint64_t unit.
> > +     * The second part start from the next uint64_t unit.
> > +     */
> > +    int part1 = 0, part2 = tot - cnt;
> > +    if (cnt % 64) {
> > +        part1 = 64 - (cnt % 64);
> > +        part2 = tot - cnt - part1;
> > +        memset(tail & ~(63ULL), 0, part1);
> > +        memset((tail + 64) & ~(63ULL), 0, part2);
> >       
> You're confusing bit and byte offsets -- cnt and tot are both byte offsets.
>     
    Yes, I will fix it.

+static inline int vext_elem_mask(void *v0, int mlen, int index)
+{
+
+    int idx = (index * mlen) / 8;
+    int pos = (index * mlen) % 8;
+
+    switch (mlen) {
+    case 8:
+        return *((uint8_t *)v0 + H1(index)) & 0x1;
+    case 16:
+        return *((uint16_t *)v0 + H2(index)) & 0x1;
+    case 32:
+        return *((uint32_t *)v0 + H4(index)) & 0x1;
+    case 64:
+        return *((uint64_t *)v0 + index) & 0x1;
+    default:

+        return (*((uint8_t *)v0 + H1(idx)) >> pos) & 0x1;
+    }

This is not what I had in mind, and looks wrong as well.

    int idx = (index * mlen) / 64;
    int pos = (index * mlen) % 64;
    return (((uint64_t *)v0)[idx] >> pos) & 1;

You also might consider passing log2(mlen), so the multiplication could be
strength-reduced to a shift.

    I don't think so. For example, when mlen is 8 bits and index is 0,
    it will reduce to
return (((uint64_t *)v0)[0]) & 1
    And it's not right. 
    
    The right bit is first bit in vector register 0. And in host big
    endianess, 
    it will be  the first bit of the seventh byte. 
> > +#define GEN_VEXT_LD_ELEM(NAME, MTYPE, ETYPE, H, LDSUF)              \
> > +static void vext_##NAME##_ld_elem(CPURISCVState *env, abi_ptr addr, \
> > +        uint32_t idx, void *vd, uintptr_t retaddr)                  \
> > +{                                                                   \
> > +    int mmu_idx = cpu_mmu_index(env, false);                        \
> > +    MTYPE data;                                                     \
> > +    ETYPE *cur = ((ETYPE *)vd + H(idx));                            \
> > +    data = "" addr, mmu_idx, retaddr);    \
> > +    *cur = data;                                                    \
> > +}                                                                   \
> >       
> If you're going to use cpu_mmu_index, you might as well use cpu_SUFF_data_ra(),
> which does not require the mmu_idx parameter.
>     
    Good.
> > +#define GEN_VEXT_ST_ELEM(NAME, ETYPE, H, STSUF)                       \
> > +static void vext_##NAME##_st_elem(CPURISCVState *env, abi_ptr addr,   \
> > +        uint32_t idx, void *vd, uintptr_t retaddr)                    \
> > +{                                                                     \
> > +    int mmu_idx = cpu_mmu_index(env, false);                          \
> > +    ETYPE data = "" *)vd + H(idx));                             \
> > +    cpu_##STSUF##_mmuidx_ra(env, addr, data, mmu_idx, retaddr);       \
> > +}
> >       
> Likewise.
>
> > +/*
> > + *** unit-stride: load vector element from continuous guest memory
> > + */
> > +static inline void vext_ld_us_mask(void *vd, void *v0, target_ulong base,
> > +        CPURISCVState *env, uint32_t desc,
> > +        vext_ld_elem_fn ld_elem,
> > +        vext_ld_clear_elem clear_elem,
> > +        uint32_t esz, uint32_t msz, uintptr_t ra)
> > +{
> > +    uint32_t i, k;
> > +    uint32_t mlen = vext_mlen(desc);
> >       
> You don't need to pass mlen, since it's
>     
    Yes.
> > +/* unit-stride: store vector element to guest memory */
> > +static void vext_st_us_mask(void *vd, void *v0, target_ulong base,
> > +        CPURISCVState *env, uint32_t desc,
> > +        vext_st_elem_fn st_elem,
> > +        uint32_t esz, uint32_t msz, uintptr_t ra)
> > +{
> > +    uint32_t i, k;
> > +    uint32_t nf = vext_nf(desc);
> > +    uint32_t mlen = vext_mlen(desc);
> > +    uint32_t vlmax = vext_maxsz(desc) / esz;
> > +
> > +    /* probe every access*/
> > +    for (i = 0; i < env->vl; i++) {
> > +        if (!vext_elem_mask(v0, mlen, i)) {
> > +            continue;
> > +        }
> > +        probe_write_access(env, base + nf * i * msz, nf * msz, ra);
> > +    }
> > +    /* store bytes to guest memory */
> > +    for (i = 0; i < env->vl; i++) {
> > +        k = 0;
> > +        if (!vext_elem_mask(v0, mlen, i)) {
> > +            continue;
> > +        }
> > +        while (k < nf) {
> > +            target_ulong addr = base + (i * nf + k) * msz;
> > +            st_elem(env, addr, i + k * vlmax, vd, ra);
> > +            k++;
> > +        }
> > +    }
> > +}
> >       
> I'll note that vext_ld_us_mask and vext_st_us_mask are identical, except for:
>
> 1) probe_read/write_access (which I already suggested merging, using
> MMUAccessType),
>
> 2) the name of the ld_elem/st_elem variable (the function types are already
> identical), and
>
> 3) the clear loop at the end of the load (which could be conditional on
> clear_elem != NULL; after inlining, this should be optimized away).
>     
    Good idea. Thanks.
    
    Zhiwei
> > +static void vext_st_us(void *vd, target_ulong base,
> > +        CPURISCVState *env, uint32_t desc,
> > +        vext_st_elem_fn st_elem,
> > +        uint32_t esz, uint32_t msz, uintptr_t ra)
> >       
> Similarly.
>
>
> r~
>