On Mon, May 09, 2016 at 03:06:18PM +0200, David Hildenbrand wrote:
Just FYI, on other architectures (e.g. s390x), other conditions (e.g. cpu generation) also define if a CPU model is runnable, so the pure availability of features does not mean that a cpu model is runnable.
We could have runnable=false and unavailable-features being an empty list.
Even on those cases, can't the root cause be mapped to a QOM property name (e.g. "cpu-generation"), even if it's property that can't be changed by the user?
In the current state, no.
But it could be implemented by s390x in the future, if it's considered useful, right?
Yes, we could fit that into read-only properties if we would ever need it (like cpu-generation you mentioned and cpu-ga-level - both will always be read-only).
However we could come up with more optional fields for that in the future. (like unsupported-values or sth. like that). I actually prefer unavailable-features over runnability-blockers.
So I think for runnable=false: a) unavailable-features set -> can be made runnable b) unavailable-features not set -> cannot be made runnable
would be enough.
I understand it would be enough, but I would like to at least define semantics that would make sense for all architectures in case it gets implemented in the future. Would the proposal below make sense?
Yes, I think so. However to really make good hints, upper layers would most likely need more information about the exact problem with a property - maybe something like an enum value per problematic property. (UNAVAILABLE_FEATURE, VALUE_TOO_BIG, VALUE_TOO_SMALL, UNSUPPORTED_VALUE) ...
If a more powerful interface is needed, we can add extra fields, yes. Although I'm not sure we really start providing that level of detail in a generic way (I guess it will depend on how much arch-independent code libvirt will use to handle runnability information).
We could replace this with something more generic, like:
@runnability-blockers: List of attributes that prevent the CPU model from running in the current host.
A list of QOM property names that represent CPU model attributes that prevent the CPU from running. If the QOM property is read-only, that means the CPU model can never run in the current host. If the property is read-write, it means that it MAY be possible to run the CPU model in the current host if that property is changed.
Management software can use it as hints to suggest or choose an alternative for the user, or just to generate meaningful error messages explaining why the CPU model can't be used.
(I am looking for a better name than "runnability-blockers").
Not sure which approach would be better.
Note that this is only a change in documentation of the new field, to allow it to cover extra cases without any changes in the interface. I also don't like the "runnability-blockers" name, but I prefer the new documentation I wrote above because it is more explicit about what exactly the field means. I plan to keep the "unavailable-features" name but use the above documentation text in the next version. Does that sound OK? -- Eduardo