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On 11/12/21 15:02, Marc Zyngier wrote:
I'd like KVM to be consistent across architectures and have the same
(similar) meaning for KVM_CAP_NR_VCPUS.
Sure, but this is a pretty useless piece of information anyway. As
Andrew pointed out, the information is available somewhere else, and
all we need to do is to cap it to the number of supported vcpus, which
is effectively a KVM limitation.

Also, we are talking about representing the architecture to userspace.
No amount of massaging is going to make an arm64 box look like an x86.

Not sure what you mean? The API is about providing a piece of information independent of the architecture, while catering for a ppc weirdness. Yes it's mostly useless if you don't care about ppc, but it's not about making arm64 look like x86 or ppc; it's about not having to special case ppc in userspace.

If anything, if KVM_CAP_NR_VCPUS returns the same for kvm and !kvm, then *that* is making an arm64 box look like an x86. On ARM the max vCPUs depends on VM's GIC configuration, so KVM_CAP_NR_VCPUS should take that into account. Or KVM_CAP_NR_VCPUS should have been only for !kvm; but the ship for that has sailed.


which I'm keen on avoiding. I'd rather have the kvm and !kvm cases
return the same thing.
Forgive me my (ARM?) ignorance but what would it be then? If we go for
min(num_online_cpus(), kvm_arm_default_max_vcpus()) in both cases, cat
this can still go above KVM_CAP_MAX_VCPUS after vGIC is created?
"min(num_online_cpus(), kvm_arm_default_max_vcpus())" is probably the
right thing in all cases. Yes, KVM_CAP_NR_VCPUS will keep reporting
more than the VM can actually support. But that's why we have
KVM_CAP_MAX_VCPUS, which tells you now many vcpus you can create for a
given configuration.

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