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

[Rob Bradford <rbradford@rivosinc.com>]

On Thu, 2024-01-11 at 11:53 -0300, Daniel Henrique Barboza wrote:
> 
> 
> 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.
> 

It seems completely reasonable to me to drop it for now.

Thanks for all the feedback,

Rob

> 
> 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