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> x86: support for Xen guests under KVM with Linux v5.12+ Clearly I haven't been keeping up, because this is a bit of a surprise. Xen under KVM? Those things a
by orra 3y ago
> x86: support for Xen guests under KVM with Linux v5.12+
Clearly I haven't been keeping up, because this is a bit of a surprise. Xen under KVM? Those things are polar opposites!
- wmf 3y agoXen is kind of dead so people are migrating to KVM but they don't want to update their images.
- luckyobserver 3y agoDoesn't AWS run on a modified Xen setup?
- monocasa 3y agoThey used to, but they run mainly on KVM now with a custom vmm.
- usr1106 3y agoFirst web hit I get on that topic is https://www.freecodecamp.org/news/aws-just-announced-a-move-from-xen-towards-kvm-so-what-is-kvm/ https://www.freecodecamp.org/news/aws-just-announced-a-move-... (I read only the headline and the date now. From Xen towards KVM starting in 2019. So I guess at AWS scale that's still a relatively new thing.)
- monocasa 3y agoThey started the transition back in 2017. https://www.theregister.com/2017/11/07/aws_writes_new_kvm_based_hypervisor_to_make_its_cloud_go_faster/ https://www.theregister.com/2017/11/07/aws_writes_new_kvm_ba...
- usr1106 3y agoCalling systemd-detect-virt on the 2 first AWS machines of different type I have access to: Result is xen on one of them and amazon on the other.
- monocasa 3y agoThis qemu support comes after Amazon added support to the kernel to emulate Xen when combined with their custom vmm. A key component of that is the hypervisor lying when it's asked by a guest to describe itself. There's a good chance that 'xen' box is actually KVM and Amazon's proprietary vmm.
- silverwind 3y agoSounds like they developed a `user-agent` situation. Apparently, one must never ask popular software what it is.
- monocasa 3y agoYou either die a hero or live long enough to see yourself become the villain (ie. last vestiges languishing as a compat shim that even the developers who support you wish they could finally deprecate and remove).
- wmf 3y agoYou're using very obsolete instances.
- usr1106 3y agoThey are neither brand new nor 10+ years old legacy types. Somewhere in the middle.
- generalizations 3y agoSince when is Xen dead? I liked the design better than kvm.
- orra 3y agoDon't know but it's a shame. IIRC Qubes uses Xen, because Xen has a small Trusted Computing Base (TCB).
- soraminazuki 3y agoXen has a smaller TCB only if you consider Dom0 not part of the TCB. But since a compromised Dom0 would lead to the compromise of the whole system, I think it makes more sense to view Dom0 as part of the TCB.
- gwd 3y agoPeople have been saying "Xen is dead" for over 15 years; and yet here we are. :-) Xen and KVM are different beasts with different advantages. (Some of Xen's I detailed in a comment here [1].) KVM in particular has some advantages in the "vanilla server virt" space, which is why KVM often ends up being the default in the Linux distro world. But we're not going away any time soon. [1] https://news.ycombinator.com/item?id=32607837 https://news.ycombinator.com/item?id=32607837
- actionfromafar 3y agoI used Xen a lot in late 2000s, when it was a great alternative to spinning up more physical machines. Had a lot of fun with the office servers, running all disks with RAID some-number-or-other on a machine with an NBD[0] server on gigabit Ethernet, and then a couple of physical machines hosting all the XEN machines. XEN felt fast and, though not exactly easy to setup, fairly straight-forward. 0: https://en.wikipedia.org/wiki/Network_block_device https://en.wikipedia.org/wiki/Network_block_device
- tjoff 3y ago[flagged]
- monocasa 3y agoSupport for Xen guests by supporting the Xen hypercalls. This was added to KVM proper to support Amazon's proprietary vmm running ancient AMIs on newer systems but it seems it's finally getting qemu support too.
- gwd 3y agoI haven't looked at this work in detail, but you need more than just hypercalls to run those ancient guests; those guests are expecting to see xenstore, disk and block backends, and so on. Those would be a lot harder to emulate in the kernel (and probably a bad idea to try anyway).
- monocasa 3y agoI mean, sure, this specific work is in qemu, not the kernel. Some hypercalls are trapped by the kernel, some trap out to a vmm like qemu. Support for pv devices mainly lives in the vmm. But A) even the hypercalls the kernel handles needs to be manually enabled by the vmm, and B) the pv device support ultimately is exposed as in guest memory tables and hypercalls (now partially emulated by qemu).
- davidjfelix 3y agoMy interpretation of this: You have an x86 machine running Linux v5.12 which is running QEMU using KVM accelerations. Using this setup, you're able to emulate a machine that can run Xen. Since Xen is a type 1 hypervisor, you would then run a Dom0 and perhaps many DomU machines inside it, let's say Linux here for both. Hardware > Linux KVM > QEMU > Xen guest > Linux Dom0 guest, Linux DomU guest
- monocasa 3y agoMy understanding is that this work is designed to run DomU images more or less directly. Qemu itself more or less fills the role of Dom0 here.
- davidjfelix 3y agoThat makes more sense.
- sureglymop 3y agoI hope so because it might finally allow me to run Qubes OS as a vm to check it out.
- bonzini 3y agoThat's nested virtualization and it's completely unrelated.
- rbanffy 3y agoI always looked at Xen as something similar to a plain VM image with lots and lots of virtio-like stuff. I'd even assume they'd perform better if they are a thinner layer of virtual machine on top of the host.