Re: [PATCH v3 3/6] target/riscv/tcg: add user flag for profile support

[Andrea Bolognani]

On Mon, Oct 23, 2023 at 07:35:16PM +0200, Andrew Jones wrote:
> On Mon, Oct 23, 2023 at 02:00:00PM -0300, Daniel Henrique Barboza wrote:
> > On 10/23/23 05:16, Andrew Jones wrote:
> > > Hmm, I'm not sure I agree with special-casing profiles like this. I think
> > > the left-to-right processing should be consistent for all. I'm also not
> > > sure we should always warn when disabling a profile. For example, if a
> > > user does
> > >
> > >   -cpu rv64,rva22u64=true,rva22u64=false
> > >
> > > then they'll get a warning, even though all they're doing is restoring the
> > > cpu model. While that looks like an odd thing to do, a script may be
> > > adding the rva22u64=true and the rva22u64=false is the user input which
> > > undoes what the script did.
> >
> > QEMU options do not work with a "the user enabled then disabled the same 
> > option,
> > thus it'll count as nothing happened" logic. The last instance of the 
> > option will
> > overwrite all previous instances. In the example you mentioned above the 
> > user would
> > disable all mandatory extensions of rva22u64 in the CPU, doesn't matter if 
> > the
> > same profile was enabled beforehand.
>
> Yup, I'm aware, but I keep thinking that we'll only be using profiles with
> a base cpu type. If you start with nothing (a base) and then add a profile
> and take the same one away, you shouldn't be taking away anything else. I
> agree that if you use a profile on some cpu type that already enabled a
> bunch of stuff itself, then disabling a profile would potentially remove
> some of those too, but mixing cpu types that have their own extensions and
> profiles seems like a great way to confuse oneself as to what extensions
> will be present.  IOW, we should be adding a base cpu type at the same
> time we're adding these profiles.

The question that keep bouncing around my head is: why would we even
allow disabling profiles?

It seems to me that it only makes things more complicated, and I
really can't see the use case for it.

Enabling additional features on top of a profile? There's obvious
value in that, so that you can model hardware that implements
optional and proprietary extensions. Enabling multiple profiles?
You've convinced me that it's useful. But disabling profiles, I just
don't see it. I believe Alistair was similarly unconvinced.

> > > As far as warnings go, it'd be nice to warn when mandatory profile
> > > extensions are disabled from an enabled profile. Doing that might be
> > > useful for debug, but users which do it without being aware they're
> > > "breaking" the profile may learn from that warning. Note, the warning
> > > should only come when the profile is actually enabled and when the
> > > extension would actually be disabled, i.e.
> > >
> > >   -cpu rv64,rva22u64=true,c=off
> > >
> > > should warn
> > >
> > >   -cpu rv64,c=off,rva22u64=true
> > >
> > > should not warn (rva22u64 overrides c=off since it's to the right)
> > >
> > >   -cpu rv64,rva22u64=true,rva22u64=false,c=off
> > >
> > > should not warn (rva22u64 is not enabled)

I think these should be hard errors, not warnings.

If you're enabling a profile and then disabling an extension that's
mandatory for that profile, you've invalidated the profile. You've
asked for a configuration that doesn't make any sense: you can't have
a CPU that both implements a profile and lacks one of its mandatory
extensions.

QEMU users could easily miss the warning. libvirt users won't see it
at all. It's a user error and it needs to be treated as such IMO.

-- 
Andrea Bolognani / Red Hat / Virtualization