Re: [PATCH 06/19] riscv: Extend cpufeature.c to detect vendor extensions

[Charlie Jenkins]

On Fri, Apr 12, 2024 at 01:30:08PM +0100, Conor Dooley wrote:
> On Thu, Apr 11, 2024 at 09:11:12PM -0700, Charlie Jenkins wrote:
> >  static void __init riscv_parse_isa_string(unsigned long *this_hwcap, struct riscv_isainfo *isainfo,
> 
> > -					  unsigned long *isa2hwcap, const char *isa)
> > +					struct riscv_isainfo *isavendorinfo, unsigned long vendorid,
> > +					unsigned long *isa2hwcap, const char *isa)
> >  {
> >  	/*
> >  	 * For all possible cpus, we have already validated in
> > @@ -349,8 +384,30 @@ static void __init riscv_parse_isa_string(unsigned long *this_hwcap, struct risc
> >  		const char *ext = isa++;
> >  		const char *ext_end = isa;
> >  		bool ext_long = false, ext_err = false;
> > +		struct riscv_isainfo *selected_isainfo = isainfo;
> > +		const struct riscv_isa_ext_data *selected_riscv_isa_ext = riscv_isa_ext;
> > +		size_t selected_riscv_isa_ext_count = riscv_isa_ext_count;
> > +		unsigned int id_offset = 0;
> >  
> >  		switch (*ext) {
> > +		case 'x':
> > +		case 'X':
> 
> One quick remark is that we should not go and support this stuff via
> riscv,isa in my opinion, only allowing it for the riscv,isa-extensions
> parsing. We don't have a way to define meanings for vendor extensions in
> this way. ACPI also uses this codepath and at the moment the kernel's
> docs say we're gonna follow isa string parsing rules in a specific version
> of the ISA manual. While that manual provides a format for the string and
> meanings for standard extensions, there's nothing in there that allows us
> to get consistent meanings for specific vendor extensions, so I think we
> should avoid intentionally supporting this here.

Getting a "consistent meaning" is managed by a vendor. If a vendor
supports a vendor extension and puts it in their DT/ACPI table it's up
to them to ensure that it works. How does riscv,isa-extensions allow for
a consistent meaning?

> 
> I'd probably go as far as to actively skip vendor extensions in
> riscv_parse_isa_string() to avoid any potential issues.
> 
> > +			bool found;
> > +
> > +			found = get_isa_vendor_ext(vendorid,
> > +						   &selected_riscv_isa_ext,
> > +						   &selected_riscv_isa_ext_count);
> > +			selected_isainfo = isavendorinfo;
> > +			id_offset = RISCV_ISA_VENDOR_EXT_BASE;
> > +			if (!found) {
> > +				pr_warn("No associated vendor extensions with vendor id: %lx\n",
> > +					vendorid);
> 
> This should not be a warning, anything we don't understand should be
> silently ignored to avoid spamming just because the kernel has not grown
> support for it yet.

Sounds good.

- Charlie

> 
> Thanks,
> Conor.
> 
> > +				for (; *isa && *isa != '_'; ++isa)
> > +					;
> > +				ext_err = true;
> > +				break;
> > +			}
> > +			fallthrough;
> >  		case 's':
> >  			/*
> >  			 * Workaround for invalid single-letter 's' & 'u' (QEMU).
> > @@ -366,8 +423,6 @@ static void __init riscv_parse_isa_string(unsigned long *this_hwcap, struct risc
> >  			}
> >  			fallthrough;
> >  		case 'S':
> > -		case 'x':
> > -		case 'X':
> >  		case 'z':
> >  		case 'Z':
> >  			/*
> > @@ -476,8 +531,10 @@ static void __init riscv_parse_isa_string(unsigned long *this_hwcap, struct risc
> >  				set_bit(nr, isainfo->isa);
> >  			}
> >  		} else {
> > -			for (int i = 0; i < riscv_isa_ext_count; i++)
> > -				match_isa_ext(&riscv_isa_ext[i], ext, ext_end, isainfo);
> > +			for (int i = 0; i < selected_riscv_isa_ext_count; i++)
> > +				match_isa_ext(&selected_riscv_isa_ext[i], ext,
> > +					      ext_end, selected_isainfo,
> > +					      id_offset);
> >  		}
> >  	}
> >  }