Lecture: lvisor walkthrough

Today’s plan

• review kernel programming
• hypervisor overview
• review exercise 1

Kernel workflow

• draw workflow: bootloader → kernel → (user ↔ kernel)*
• tests/lv6/head.S: bootstrapping
• tests/lv6/main.c: kernel & user setup
  • kernel/trap.c & kernel/idt.c
    • GDT/IDT/TSS
    • LSTAR
    • %gs
  • kernel/entry.S
    • syscall fast path
    • exception/interrupt
    • how registers are saved
• tests/lv6/syscall.c: system calls

hypervisor workflow

• workflow
  • Intel SDM 23.4, Figure 23-1: Interaction of a Virtual-Machine Monitor and Guests
  • vmxon → (VM-entry ↔ VM-exit)* → vmxoff
  • see vmm/kvm.c (more on EPT later)
• VMCS: virtual-machine control structure
  • key data structure to control virtual machines
    • guest state
    • host state
    • control fields: VM-exectuion, VM-exit, VM-entry
    • VM-exit information
  • state machine: SDM 24.1, Figure 24-1: States of VMCS X
    • vmptrld, vmlaunch, vmresume, vmclear
    • vmread, vmwrite
  • VMCB on AMD
• vmm/vmx.c
  • vmx_cpu_has_kvm_support: check if hardware supports vmx
  • vmx_disabled_by_bios: check if vmx is disabled by BIOS
  • vmx_vcpu_enable: enter root mode via vmxon
  • vmx_vcpu_setup: initialize vmcs
  • vmx_vcpu_run: VM-entry and VM-exit
    • what registers are saved by the cpu (in VMCS)
    • what registers are saved by software (in struct kvm_vcpu)
  • vmx_handle_exit: read exit rason & dispatch to VM-exit handlers
  • vmx_skip_emulated_instruction: skip current instruction in guest
• guest bootstrapping firmware/*
  • draw physical memory map: guest kernel, vmm
  • one approach is to emulate a real machine
  • a simpler/faster way: communicate with the host (see guest_params)
• summary
  • power-on → bootstrapping → OS/hypervisor
  • → vmxon → guest power-on → guest bootstrapping → (guest OS ↔ hypervisor)*