Re: [PATCH v3 06/23] xen/riscv: introduce guest riscv,isa string

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 6/22/26 4:09 PM, Jan Beulich wrote:
> On 17.06.2026 13:17, Oleksii Kurochko wrote:
> > Introduce generation of the riscv,isa string passed to the guest via the
> > Device Tree riscv,isa property.
> >
> > Introduce the per-domain isa string and guest isa bitmap, populated
> > during domain creation by calling init_guest_isa().
> >
> > Introduce guest_unsupp to filter out ISA extensions that should not be
> > exposed to guests:
> >
> > - f/d/q/v: FPU and vector context save/restore are not yet implemented
> >    for guests.
> I may have asked before - what about Zfinx, Zdinx (and the supposed Zqinx)?
> They aren't in riscv_isa_ext[], yes, but perhaps wrongly so? And hence they
> may want at least mentioning?
They are not supported by Xen so they aren't in riscv_isa_ext so it 
looks fine for me.
They are not in guest_unsupp as they aren't present in riscv_isa_ext and 
so it won't be propagated to guest anyway because of:
  +    bitmap_andnot(d->arch.isa, riscv_isa, guest_unsupp,
  +                  RISCV_ISA_EXT_MAX);

While it isn't in riscv_isa_ext[] I think it is fine not to add them to 
guest_unsupp, so I will add to the commit message that:
```
- Zfinx, Zdinx and Zqinx are not implemented for guests either; as they 
are not present in the riscv_isa_ext[] array, they can never be set in 
riscv_isa and thus are never exposed to a guest, so there is no need to 
list them explicitly in guest_unsupp.
```

I think it is fine for now but probably it will need to be reworked in 
future.

> > @@ -480,6 +489,78 @@ bool riscv_isa_extension_available(const unsigned long 
> > *isa_bitmap,
> >       return test_bit(id, isa_bitmap);
> >   }
> >   
> > +static int build_guest_isa_str(char *buf, size_t size,
> > +                               const unsigned long *isa_bitmap)
> > +{
> > +    int total;
> > +
> > +#if defined(CONFIG_RISCV_32)
> > +    total = snprintf(buf, size, "rv32");
> > +#elif defined(CONFIG_RISCV_64)
> > +    total = snprintf(buf, size, "rv64");
> > +#else
> > +#   error "Unsupported RISC-V bitness"
> > +#endif
> > +
> > +    if ( total < 0 )
> > +        return total;
> > +
> > +    for ( unsigned int i = 0; i < ARRAY_SIZE(riscv_isa_ext); i++ )
> > +    {
> > +        const struct riscv_isa_ext_data *ext = &riscv_isa_ext[i];
> > +        int ret;
> > +
> > +        if ( !riscv_isa_extension_available(isa_bitmap, ext->id) )
> > +            continue;
> > +
> > +        ret = snprintf(buf ? buf + total : NULL,
> > +                       buf ? size - total : 0, "%s%s",
> If total > size this subtraction will underflow and a huge value will be
> passed to snprintf().
I will add the check before for():

if ( buf && ((size_t)total >= size) )
    return -ENOSPC;

Thanks.

~ Oleksii