Re: [PATCH v2 00/10] riscv: RVA22U64 profile support

[Alistair Francis <alistair23@gmail.com>]

On Mon, Oct 16, 2023 at 7:03 PM Andrew Jones <ajones@ventanamicro.com> wrote:
>
> On Thu, Oct 12, 2023 at 04:07:50PM -0300, Daniel Henrique Barboza wrote:
> >
> >
> > On 10/11/23 00:01, Alistair Francis wrote:
> > > On Sat, Oct 7, 2023 at 12:23 AM Daniel Henrique Barboza
> > > <dbarboza@ventanamicro.com> wrote:
> > > >
> > > > Hi,
> > > >
> > > > Several design changes were made in this version after the reviews and
> > > > feedback in the v1 [1]. The high-level summary is:
> > > >
> > > > - we'll no longer allow users to set profile flags for vendor CPUs. If
> > > >    we're to adhere to the current policy of not allowing users to enable
> > > >    extensions for vendor CPUs, the profile support would become a
> > > >    glorified way of checking if the vendor CPU happens to support a
> > > >    specific profile. If a future vendor CPU supports a profile the CPU
> > > >    can declare it manually in its cpu_init() function, the flag will
> > > >    still be set, but users can't change it;
> > > >
> > > > - disabling a profile will now disable all the mandatory extensions from
> > > >    the CPU;
> > >
> > > What happens if you enable one profile and disable a different one?
> >
> > With this implementation as is the profiles will be evaluated by the order they're
> > declared in riscv_cpu_profiles[]. Which isn't exactly ideal since we're exchanging
> > a left-to-right ordering in the command line by an arbitrary order that we happened
> > to set in the code.
> >
> > I can make some tweaks to make the profiles sensible to left-to-right order between
> > them, while keeping regular extension with higher priority. e.g.:
> >
> >
> > -cpu rv64,zicbom=true,profileA=false,profileB=true,zicboz=false
> > -cpu rv64,profileA=false,zicbom=true,zicboz=false,profileB=true
> > -cpu rv64,profileA=false,profileB=true,zicbom=true,zicboz=false
> >
> > These would all do the same thing: "keeping zicbom=true and zicboz=false, disable profileA
> > and then enable profile B"
> >
> > Switching the profiles order would have a different result:
> >
> > -cpu rv64,profileB=true,profileA=false,zicbom=true,zicboz=false
> >
> > "keeping zicbom=true and zicboz=false, enable profile B and then disable profile A"
> >
> >
> > I'm happy to hear any other alternative/ideas. We'll either deal with some left-to-right
> > ordering w.r.t profiles or deal with an internal profile commit ordering. TBH I think
> > it's sensible to demand left-to-right command line ordering for profiles only.
>
> left-to-right ordering is how the rest of QEMU properties work and scripts
> depend on it. For example, one can do -cpu $MODEL,$DEFAULT_PROPS,$MORE_PROPS
> where $MORE_PROPS can not only add more props but also override default
> props (DEFAULT_PROPS='foo=off', MORE_PROPS='foo=on' - foo will be on).
> left-to-right also works with multiple -cpu parameters, i.e. -cpu
> $MODEL,$DEFAULT_PROPS -cpu $MODEL,$MY_PROPS will replace default props
> with my props.

That seems like the way to go then

>
> I don't think profiles should be treated special with regard to this. They
> should behave the same as any property. If one does
> profileA=off,profileB=on and there are overlapping extensions then a

But what does this mean? What intent is the user saying here?

For example if a user says:

    RVA22U64=off,RVA24U64=on

They only want the extensions that were added in RVA24U64? What about
G and the standard extensions?

To me it just seems really strange to have more than 1 profile.
Profiles are there to help software and users have common platforms.
Why would a user want to mix-n-match them

Alistair

> sanity check in cpu-finalize should catch that and error out. Otherwise,
> why not. Profiles are just like big 'G' extensions and 'G' would behave
> the same way.
>
> Thanks,
> drew