Virtual Machine Reqs:
1 Efficient (run on real hardware)
2  Accurate (allow un-changed OS code)
3    Safety (protect host from guest)

3 requires input from Intel

"Sensitive" instruction...
- Behavior is different in kernel mode
  from user mode
- X86 Rings: 0 (kernel), 1, 2, 3 (user)
- Instr: "Switch ring to 3"
- Instr: "Turn off interrupts"
-- Ring 0: Really change a flag (stop listen)
-- Ring 3: NOP (actual X86 pre 2005)
-- Ring 3: Trap (X86 post 2005)

Privileged Instruction...
- Instructions that trap outside of ring 0

Sensitive ⊆ Privileged => Efficient Virt is pos.

VMware came out in 1999. Started in 1994
-- Used to do binary rewriting (and still do)

Paravirtualization
- VM not of X86 but of fake CPU
- Xen

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IBM 1972
1974 theory of VMs 

OS/360 - hardware + software
OS/370 - new hardware + new software + VM/360

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Type 1 - Machine > Hypervisor > Guests
Type 2 - Machine > Host > VM > Guests