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One can certainly write software that virtualizes another machine. Sure, performance might not be great. Sure, it's better if you have an implementation in sili

One can certainly write software that virtualizes another machine. Sure, performance might not be great. Sure, it's better if you have an implementation in silicon. But that doesn't prevent making a demo of a software VM running another kernel. What am I missing?

saagarjha 6y ago
I would consider that a fraudulent demo, since it is selling something different from what it is showing.
- scoopertrooper 6y ago
  No? There was nothing in the context of that video to suggest it was running on an A12Z. They were showing you the features that will be available on the chip they're going to bring to market sometime this year. This chip must already exist in a near-production form in order to meet that deadline.
  - saagarjha 6y ago
    I’m not saying they did, I’m pretty sure they were running it on an undisclosed chip. What would be fraudulent would be emulating it on the A12Z and claiming they were virtualizing.
- jki275 6y ago
  Apple isn't selling anything at the moment. It was only an announcement, and it remains to be seen if what they deliver matches what they announced.
waon 6y ago
Not sure what you mean by "software VMs," but if you mean CPU emulators, they're fundamentally different from VMs. Emulation is not "virtualization."
- soganess 6y ago
  No judgement of not knowing but para-virtualization is not emulation and it is not a new or niche technology either. I think IBM was doing it in the 60s (might be off base with that factoid because modern-ish operating systems like the RC4000 weren't even a thing until the late 60s). It has simply been superseded by hardware assisted vt that allows for multiple "ring-0" guests. There is a greater overhead and a limited scope when using paravirtualization. But that doesn't mean it is a relic, in fact you can try it right now in Virtualbox. I also believe that the Linux KVM has para-virtualization optimizations if the guest is Linux and the bare metal doesn't support hardware VT.
  - waon 6y ago
    I'm confused, are you sure you've responded to the right comment? No one is talking about paravirtualization in this thread. As I take it, the parent comment was suggesting that Apple was faking virtualization with some kind of software trick in the demo, which isn't what paravirtualization is about. Paravirtualization is a general optimization technique for VMs that allows the guest OS to better communicate its intent to the hypervisor through the use of hypercall APIs, sparing the guest OS from having to issue many series of privileged CPU instructions that each have to be trapped and handled by the hypervisor. It saves time because going back and forth between the hypervisor and the guest OS is an expensive operation, and reducing the number of times it happens helps a lot. It's not a simple replacement for hardware assisted virtualization or the other way around.
    - fanf2 6y ago
      Paravirtualization is relevant if the CPU does not support full virtualization - Xen was originally designed to use PV because it was very difficult to make x86 VMs fast because unmodified guest OSs required slow emulation. See section 2 of https://www.cl.cam.ac.uk/research/srg/netos/papers/2003-xensosp.pdf https://www.cl.cam.ac.uk/research/srg/netos/papers/2003-xens...
      - waon 6y ago

saagarjha 6y ago

I would consider that a fraudulent demo, since it is selling something different from what it is showing.

scoopertrooper 6y ago
No? There was nothing in the context of that video to suggest it was running on an A12Z. They were showing you the features that will be available on the chip they're going to bring to market sometime this year. This chip must already exist in a near-production form in order to meet that deadline.
- saagarjha 6y ago
  I’m not saying they did, I’m pretty sure they were running it on an undisclosed chip. What would be fraudulent would be emulating it on the A12Z and claiming they were virtualizing.
jki275 6y ago
Apple isn't selling anything at the moment. It was only an announcement, and it remains to be seen if what they deliver matches what they announced.

waon 6y ago

Not sure what you mean by "software VMs," but if you mean CPU emulators, they're fundamentally different from VMs. Emulation is not "virtualization."

soganess 6y ago
No judgement of not knowing but para-virtualization is not emulation and it is not a new or niche technology either. I think IBM was doing it in the 60s (might be off base with that factoid because modern-ish operating systems like the RC4000 weren't even a thing until the late 60s). It has simply been superseded by hardware assisted vt that allows for multiple "ring-0" guests. There is a greater overhead and a limited scope when using paravirtualization. But that doesn't mean it is a relic, in fact you can try it right now in Virtualbox. I also believe that the Linux KVM has para-virtualization optimizations if the guest is Linux and the bare metal doesn't support hardware VT.
- waon 6y ago
  I'm confused, are you sure you've responded to the right comment? No one is talking about paravirtualization in this thread. As I take it, the parent comment was suggesting that Apple was faking virtualization with some kind of software trick in the demo, which isn't what paravirtualization is about. Paravirtualization is a general optimization technique for VMs that allows the guest OS to better communicate its intent to the hypervisor through the use of hypercall APIs, sparing the guest OS from having to issue many series of privileged CPU instructions that each have to be trapped and handled by the hypervisor. It saves time because going back and forth between the hypervisor and the guest OS is an expensive operation, and reducing the number of times it happens helps a lot. It's not a simple replacement for hardware assisted virtualization or the other way around.
  - fanf2 6y ago
    Paravirtualization is relevant if the CPU does not support full virtualization - Xen was originally designed to use PV because it was very difficult to make x86 VMs fast because unmodified guest OSs required slow emulation. See section 2 of https://www.cl.cam.ac.uk/research/srg/netos/papers/2003-xensosp.pdf https://www.cl.cam.ac.uk/research/srg/netos/papers/2003-xens...
    - waon 6y ago