Re: [PATCH 3/3] target/riscv: Enable 'B' extension on max CPU type

[Daniel Henrique Barboza]

On 1/11/24 10:02, Andrew Jones wrote:
> On Wed, Jan 10, 2024 at 03:32:21PM -0300, Daniel Henrique Barboza wrote:
> > On 1/9/24 14:07, Rob Bradford wrote:
> > > Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
> > > ---
> > >    target/riscv/tcg/tcg-cpu.c | 3 ++-
> > >    1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/target/riscv/tcg/tcg-cpu.c b/target/riscv/tcg/tcg-cpu.c
> > > index f10871d352..9705daec93 100644
> > > --- a/target/riscv/tcg/tcg-cpu.c
> > > +++ b/target/riscv/tcg/tcg-cpu.c
> > > @@ -999,7 +999,8 @@ static void riscv_init_max_cpu_extensions(Object *obj)
> > >        const RISCVCPUMultiExtConfig *prop;
> > >        /* Enable RVG, RVJ and RVV that are disabled by default */
> > > -    riscv_cpu_set_misa(env, env->misa_mxl, env->misa_ext | RVG | RVJ | RVV);
> > > +    riscv_cpu_set_misa(env, env->misa_mxl,
> > > +                       env->misa_ext | RVG | RVJ | RVV | RVB);
> >
> > I'm aware that we decided a while ago the 'max' CPU could only have non-vendor 
> > and
> > non-experimental extensions enabled. RVB is experimental, so in theory we 
> > shouldn't
> > enable it.
> >
> > But RVB is an alias for zba, zbb and zbs, extensions that the 'max' CPU is 
> > already
> > enabling. In this case I think it's sensible to enable RVB here since it would 
> > just                                                                            
> >                          
> > reflect stuff that it's already happening.
>
> It's also setting the B bit in misa, which, until this spec is at least
> frozen, is a reserved bit and reserved bits "must return zero when read".

This is a side effect that I wasn't aware of.

Rob, given that the 'max' CPU already has the zb* extensions enabled, is there 
any
other gain in enabling RVB in this CPU? If there isn't I'd rather leave this one
out for now.


Thanks,

Daniel


> I don't want to stand in the way of progress and it seems 99.9% likely
> that the spec will be frozen and ratified, but, if we want to stick to
> our policies (which we should document), then even the 'max' cpu type
> should require x-b be added to the command line if it wants the B bit
> set in misa.
>
> Thanks,
> drew