Re: [PATCH v3 02/14] util: Add RISC-V vector extension probe in cpuinfo

[LIU Zhiwei]

On 2024/9/9 23:45, Richard Henderson wrote:
> On 9/9/24 00:18, LIU Zhiwei wrote:
> > On 2024/9/5 11:34, Richard Henderson wrote:
> > > On 9/4/24 07:27, LIU Zhiwei wrote:
> > > > +    if (info & CPUINFO_ZVE64X) {
> > > > +        /*
> > > > +         * Get vlenb for Vector: vsetvli rd, x0, e64.
> > > > +         * VLMAX = LMUL * VLEN / SEW.
> > > > +         * The "vsetvli rd, x0, e64" means "LMUL = 1, SEW = 64, rd 
> > > > = VLMAX",
> > > > +         * so "vlenb = VLMAX * 64 / 8".
> > > > +         */
> > > > +        unsigned long vlmax = 0;
> > > > +        asm volatile(".insn i 0x57, 7, %0, zero, (3 << 3)" : 
> > > > "=r"(vlmax));
> > > > +        if (vlmax) {
> > > > +            riscv_vlenb = vlmax * 8;
> > > > +            assert(riscv_vlen >= 64 && !(riscv_vlen & (riscv_vlen 
> > > > - 1)));
> > > > +        } else {
> > > > +            info &= ~CPUINFO_ZVE64X;
> > > > +        }
> > > > +    }
> > >
> > > Surely this does not compile, since the riscv_vlen referenced in the 
> > > assert does not exist.
> > riscv_vlen is macro about riscv_vlenb. I think you miss it.
>
> I did miss the macro.  But there's also no need for it to exist.
>
> > > That said, I've done some experimentation and I believe there is a 
> > > further simplification to be had in instead saving log2(vlenb).
> > >
> > >     if (info & CPUINFO_ZVE64X) {
> > >         /*
> > >          * We are guaranteed by RVV-1.0 that VLEN is a power of 2.
> > >          * We are guaranteed by Zve64x that VLEN >= 64, and that
> > >          * EEW of {8,16,32,64} are supported.
> > >          *
> > >          * Cache VLEN in a convenient form.
> > >          */
> > >         unsigned long vlenb;
> > >         asm("csrr %0, vlenb" : "=r"(vlenb));
> >
> > Should we use the .insn format here? Maybe we are having a compiler 
> > doesn't support vector.
>
> Neither gcc nor clang requires V be enabled at compile time in order 
> to access the CSR.
> It does seem like a mistake, but I'm happy to use it.

Can we follow you here? 🙂

Zhiwei

> r~