Re: Re: Re: [PATCH 1/3] target/riscv: Add infrastructure for 'B' MISA extension

[Andrew Jones]

On Thu, Jan 11, 2024 at 03:17:25PM +0000, Rob Bradford wrote:
> + Ved
> 
> On Thu, 2024-01-11 at 14:14 +0100, Andrew Jones wrote:
> > On Thu, Jan 11, 2024 at 02:07:34PM +0100, Andrew Jones wrote:
> > > On Tue, Jan 09, 2024 at 05:07:35PM +0000, Rob Bradford wrote:
> > > > Add the infrastructure for the 'B' extension which is the union
> > > > of the
> > > > Zba, Zbb and Zbs instructions.
> > > > 
> > > > Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
> > > > ---
> > > >  target/riscv/cpu.c         | 5 +++--
> > > >  target/riscv/cpu.h         | 1 +
> > > >  target/riscv/tcg/tcg-cpu.c | 1 +
> > > >  3 files changed, 5 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> > > > index b07a76ef6b..22f8e527ff 100644
> > > > --- a/target/riscv/cpu.c
> > > > +++ b/target/riscv/cpu.c
> > > > @@ -38,9 +38,9 @@
> > > >  #include "tcg/tcg.h"
> > > >  
> > > >  /* RISC-V CPU definitions */
> > > > -static const char riscv_single_letter_exts[] = "IEMAFDQCPVH";
> > > > +static const char riscv_single_letter_exts[] = "IEMAFDQCBPVH";
> > > 
> > > Is there a corresponding proposed change to table 29.1 of the
> > > nonpriv spec
> > > which states B comes after C and before P? If so, can you provide a
> > > link
> > > to it? Otherwise, how do we know that?
> > 
> > Oh, I see. The unpriv spec B chapter comes after the C chapter (and
> > before
> > J, P, ...). I still wonder if we'll have a 29.1 table update with the
> > ratification of this extension though.
> > 
> > 
> 
> I agree it's a bit confusing - but the order is established by the
> table in the unprivileged spec and the table explanation also makes
> this clear.
> 
> """
> Table 27.1: Standard ISA extension names. The table also defines the
> canonical order in which
> extension names must appear in the name string, with top-to-bottom in
> table indicating first-to-last
> in the name string, e.g., RV32IMACV is legal, whereas RV32IMAVC is not.
> """

Yes, this is the table I was referring to when I referenced "table 29.1 of
the nonpriv spec". Since there's a chance I was looking at too old a spec
I've now gone straight to the source,

https://github.com/riscv/riscv-isa-manual/blob/main/src/naming.adoc

but I still don't see B there. Do you see B in the table you're looking
at?

> 
> The proposed B specification does not make any remarks about the
> ordering in the ISA definition string. [1] I would worry there would be
> a lot of software churn if this ordering were to be changed.

The ordering shouldn't change, but I can't see where it's documented
(beyond the B chapter coming after the C chapter).

Thanks,
drew