On Thu, 2021-01-28 at 13:18 +0100, Paolo Bonzini wrote:
> My only qualm is really that the userspace API is really ugly.
> 
> Can you just have both a VM and a VCPU ioctl (so no vcpu_id to pass!), 

Sure, that seems like a sensible thing to do.

> add a generous padding to the struct,

I think I added *some* padding to the struct kvm_xen_hvm_attr which
wasn't there in Joao's original. I can add more, certainly.

>  and just get everything out with a 
> single ioctl without having to pass in a type?

Honestly, I don't even care about reading it out except for long_mode
which the kernel *does* infer for itself when the MSR is used to fill
in the hypercall page.

I quite like keeping them separate; they *do* get set separately, in
response to different hypercalls from the guest. And the capabilities
translate naturally to a given field existing or not existing; having
another mapping of that to fields in a binary structure would be
additional complexity.