feat(libstore): warn when kvm is enabled but /dev/kvm isn't available

[lovesegfault]

Motivation

I had a host with the kvm system-feature enabled, but no /dev/kvm available and it was causing VM tests to be extremely slow. It took me a minute to figure out that the host didn't have KVM, and that qemu was falling back to emulation. I figured it'd be nice to warn in this scenario.

Context

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[edolstra]

I think the real issue here is that system-features shouldn't contain kvm if /dev/kvm doesn't exist. Nix does that correctly (see Settings::getDefaultSystemFeatures()) but NixOS sets it by default in nix.conf.

[lovesegfault]

@edolstra this wasn't on a NixOS system at all, but rather a (ubuntu) container image with Nix that had KVM enabled in the config, but was deployed to a host without virtualization support.

I don't think it's bad that you can enable kvm on a host without it and let qemu fallback to emulation, it's handy if you don't have a host with KVM available, but doing it silently is just a bit surprising.

[Mic92]

@edolstra this wasn't on a NixOS system at all, but rather a (ubuntu) container image with Nix that had KVM enabled in the config, but was deployed to a host without virtualization support.

I don't think it's bad that you can enable kvm on a host without it and let qemu fallback to emulation, it's handy if you don't have a host with KVM available, but doing it silently is just a bit surprising.

Sounds reasonable to me.

[edolstra]

I don't think it's bad that you can enable kvm on a host without it and let qemu fallback to emulation, it's handy if you don't have a host with KVM available, but doing it silently is just a bit surprising.

You can still manually add it to system-features, it just shouldn't be the default. That defeats the purpose of the kvm system feature.

[Mic92]

I don't think it's bad that you can enable kvm on a host without it and let qemu fallback to emulation, it's handy if you don't have a host with KVM available, but doing it silently is just a bit surprising.

You can still manually add it to system-features, it just shouldn't be the default. That defeats the purpose of the kvm system feature.

Ok. This is fair, but is this not somewhat orthogonal to having this warning if someone misconfigures their nix?

[Ericson2314]

Are people using kvm in Nixpkgs to control whether to use QEMU at all?