Messages in this thread | | | Date | Fri, 12 Nov 2021 15:10:41 +0100 | Subject | Re: [PATCH 1/5] KVM: arm64: Cap KVM_CAP_NR_VCPUS by KVM_CAP_MAX_VCPUS | From | Paolo Bonzini <> |
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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.
Paolo
>>> 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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