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

[Andrew Jones]

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.

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
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