5 ms·

I have ever only heard KVM in the context of a Keyboard Video Monitor-type device but somehow I can't fully fit that into the concept of a virtual machine. Does

by Y-bar 3y ago

I have ever only heard KVM in the context of a Keyboard Video Monitor-type device but somehow I can't fully fit that into the concept of a virtual machine. Does it mean something different here?

viraptor 3y ago
https://linux-kvm.org/page/Main_Page https://linux-kvm.org/page/Main_Page
- nonrandomstring 3y ago
  Is there a connection to User Mode Linux (UML) from around the same time? Or are these completely unrelated projects? I get that running a kernel in the user space provided by another kernel is not really the same as a proper hypervisor, but have never really dug deep into why and what the various tradeoffs are.
  - blitzclone 3y ago
    There is no real connection to UML here. Hardware virtualization (Intel VT, AMD-V) are much faster in practice and also don't require the guest operating system to be heavily modified. So besides as curiosity or test vehicle, approaches like UML are pretty dead.
    - nonrandomstring 3y ago
      Thanks. So, slow as it may be, the win for UML (which seems to still have a heartbeat) is that it can run on uP without any specific virtualisation capabilities, right? If I could run Linux on a Z80/6502 then in theory I could run a virtualised Linux on a Z80/6502.
      - mark_undoio 3y ago
        Yes - plus the original win of UML was also being able to run virtual instances on a kernel without proper virtualization capabilities. In the early 2000s people used to use UMLs as a hosting platform - they didn't have the same security isolation as a proper VM (or even, necessarily, of a container) though.
        als0 3y ago
        How do containers have better security isolation than UML?
        nonrandomstring 3y ago
        When I tinkered with UML I think it was prior to cgroups (2007) [0] so my guess is that escaping the UML instance was easier. [0] https://en.wikipedia.org/wiki/Cgroups https://en.wikipedia.org/wiki/Cgroups
      - deleted 3y ago
        [deleted]
    - ComputerGuru 3y ago
      The “original” UML is/was, I believe, NetBSD running as a “rump kernel” and something that virtualization of the actual kernel does not, directly and on its own, fill the shoes of.
      - monocasa 3y ago
        UML is older than NetBSD rump kernels.
        ComputerGuru 3y ago
        I stand corrected.
  - actionfromafar 3y ago
    I want to know what Linux a386 was. Couldn't ever really understand what it did.
izacus 3y ago
It means Kernel-Based Virtual Machine, a VM engine dating back to 2007: https://en.wikipedia.org/wiki/Kernel-based_Virtual_Machine https://en.wikipedia.org/wiki/Kernel-based_Virtual_Machine
deleted 3y ago
[deleted]
szszrk 3y ago
Yes, quite different. It's basically a project that allows you to use Linux as a hypervisor. A very popular project. https://en.m.wikipedia.org/wiki/Kernel-based_Virtual_Machine https://en.m.wikipedia.org/wiki/Kernel-based_Virtual_Machine
- Y-bar 3y ago
  Thanks!
  - cloudwalk9 3y ago
    Funny enough, a KVM might be needed for KVM if you give the VM control of a USB hub and a discrete GPU using IOMMU and you only have one monitor and set of peripherals, but two hubs and an integrated GPU.
kitd 3y ago
Happy 10000 Day! https://xkcd.com/1053/ https://xkcd.com/1053/
nyrikki 3y ago
While an over simplification, here is the context There are two large hypervisors in the Linux world. Xen, which extends the kernel to support virtual CPUs with time slices. KVM, which assigns each virtual core a process that uses the Linux scheduler. When a hardware vm vcpu core is preempted there is vmexit call that has to reset registers etc... and it is expensive. Xen is what legacy AWS instances ran on and has advantages for being fair to guests is an easier task. KVM has the advantage of gaining the benefits of the Linux scheduler which is red black tree based and well optimized. When a new CPU comes out for example, KVM gains support from the upstream while Xen has to support it themselves. Once technology like cgroups improved the benefits of letting your thread complete and not be preempted due to the time slice expiring avoided the cost of vmexit. In theory, leveraging the inherently optimized core Linux features is what will also benefit virtualbox. Most people who use KVM are using an abstraction layer like libvirt that hides how it is implemented. In fact if you look at the processes you will see qemu even if KVM is how it is implemented.

by Y-bar 3y ago

I have ever only heard KVM in the context of a Keyboard Video Monitor-type device but somehow I can't fully fit that into the concept of a virtual machine. Does it mean something different here?

viraptor 3y ago

https://linux-kvm.org/page/Main_Page https://linux-kvm.org/page/Main_Page

nonrandomstring 3y ago
Is there a connection to User Mode Linux (UML) from around the same time? Or are these completely unrelated projects? I get that running a kernel in the user space provided by another kernel is not really the same as a proper hypervisor, but have never really dug deep into why and what the various tradeoffs are.
- blitzclone 3y ago
  There is no real connection to UML here. Hardware virtualization (Intel VT, AMD-V) are much faster in practice and also don't require the guest operating system to be heavily modified. So besides as curiosity or test vehicle, approaches like UML are pretty dead.
  - nonrandomstring 3y ago
    Thanks. So, slow as it may be, the win for UML (which seems to still have a heartbeat) is that it can run on uP without any specific virtualisation capabilities, right? If I could run Linux on a Z80/6502 then in theory I could run a virtualised Linux on a Z80/6502.
    - mark_undoio 3y ago
      Yes - plus the original win of UML was also being able to run virtual instances on a kernel without proper virtualization capabilities. In the early 2000s people used to use UMLs as a hosting platform - they didn't have the same security isolation as a proper VM (or even, necessarily, of a container) though.
      - als0 3y ago
        How do containers have better security isolation than UML?
        nonrandomstring 3y ago
        When I tinkered with UML I think it was prior to cgroups (2007) [0] so my guess is that escaping the UML instance was easier. [0] https://en.wikipedia.org/wiki/Cgroups https://en.wikipedia.org/wiki/Cgroups
    - deleted 3y ago
      [deleted]
  - ComputerGuru 3y ago
    The “original” UML is/was, I believe, NetBSD running as a “rump kernel” and something that virtualization of the actual kernel does not, directly and on its own, fill the shoes of.
    - monocasa 3y ago
      UML is older than NetBSD rump kernels.
      - ComputerGuru 3y ago
        I stand corrected.
- actionfromafar 3y ago
  I want to know what Linux a386 was. Couldn't ever really understand what it did.

izacus 3y ago

It means Kernel-Based Virtual Machine, a VM engine dating back to 2007: https://en.wikipedia.org/wiki/Kernel-based_Virtual_Machine https://en.wikipedia.org/wiki/Kernel-based_Virtual_Machine

deleted 3y ago

[deleted]

szszrk 3y ago

Yes, quite different. It's basically a project that allows you to use Linux as a hypervisor. A very popular project. https://en.m.wikipedia.org/wiki/Kernel-based_Virtual_Machine https://en.m.wikipedia.org/wiki/Kernel-based_Virtual_Machine

Y-bar 3y ago
Thanks!
- cloudwalk9 3y ago
  Funny enough, a KVM might be needed for KVM if you give the VM control of a USB hub and a discrete GPU using IOMMU and you only have one monitor and set of peripherals, but two hubs and an integrated GPU.

kitd 3y ago

Happy 10000 Day! https://xkcd.com/1053/ https://xkcd.com/1053/

nyrikki 3y ago

While an over simplification, here is the context There are two large hypervisors in the Linux world. Xen, which extends the kernel to support virtual CPUs with time slices. KVM, which assigns each virtual core a process that uses the Linux scheduler. When a hardware vm vcpu core is preempted there is vmexit call that has to reset registers etc... and it is expensive. Xen is what legacy AWS instances ran on and has advantages for being fair to guests is an easier task. KVM has the advantage of gaining the benefits of the Linux scheduler which is red black tree based and well optimized. When a new CPU comes out for example, KVM gains support from the upstream while Xen has to support it themselves. Once technology like cgroups improved the benefits of letting your thread complete and not be preempted due to the time slice expiring avoided the cost of vmexit. In theory, leveraging the inherently optimized core Linux features is what will also benefit virtualbox. Most people who use KVM are using an abstraction layer like libvirt that hides how it is implemented. In fact if you look at the processes you will see qemu even if KVM is how it is implemented.