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
