Introduction to Virtualization 0

# Introduction to Virtualization 0 #

By SilentMing

Blog: http://silentming.net

[Slides](https://silentming.net/BlogExamples/virtualization-intro/virtualization-intro-0-slides.html#1)

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## Story Line ##

* Brief introduction to virtualization
 * (Definition, histroy ...)

* Classification
 * (Spectrum)

* Architecture on ARM (Type-2 Hypervisor)
 * (It's simple, but only cover partial)

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## What is Virtualization ##

**Virtualization:** A layer mapping its visible interface and resources onto the interface and
resources of the underlying layer or system on which it is implemented

- Abstraction – to simplify the use of the underlying resource (e.g., by removing details of the resource’s structure)

- Replication – to create multiple instances of the resource (e.g., to simplify management or allocation)

- Isolation – to separate the uses which clients make of the underlying resources (e.g., to improve security)

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## How Dose It Come ? ##

Run **multiple OSes** on a **Single** Computer

Histroy

- 1960: [CP-40](https://en.wikipedia.org/wiki/IBM_CP-40) - full virtualization
- 1970: [System/370](https://en.wikipedia.org/wiki/IBM_System/370) - without virtual memory, time-sharing
- 1999: [VMware Virtual Platform](https://en.wikipedia.org/wiki/X86_virtualization) - x86 virtualization support
- 2003: [Xen and the Art of Virtualization](http://www.cs.yale.edu/homes/yu-minlan/teach/csci599-fall12/papers/xen.pdf) - Para-virtualization
- 2005: [QEMU(Quick Emulator)](https://www.usenix.org/legacy/event/usenix05/tech/freenix/full_papers/bellard/bellard_html/) -  rescue for those times when VMware is overkill
- 2005: Free Desktop Virtualization - VMWare
- 2005 & 2006: [Intel VT-x & AMD-V](https://en.wikipedia.org/wiki/X86_virtualization) (Hardware support for virtualization)
- 2007: [KVM(Kernel-based Virtual Machine)](https://ols.fedoraproject.org/OLS/Reprints-2007/OLS2007-Proceedings-V1.pdf#page=225)
- 2011: [ARMv8](https://www.arm.com/files/downloads/ARMv8_white_paper_v5.pdf) virtualization support
- 2013: [QEMU 1.5] ARM Cortex-A9 & Cortex-A15
- 2014: [KVM/ARM](https://dl.acm.org/citation.cfm?id=2541946) & [Xen 4.4](https://wiki.xenproject.org/wiki/Xen_Project_4.4_Feature_List)
- 2017: [ARMv8.4: VHE(Virtualization Host Extension)](https://lwn.net/Articles/650524/)

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## Why Virtualization ##

#### Consolidation
- Run several different OS on a single machine

#### Isolation
- Keep the VMs separated as error container
- Fault tolerant

#### Maintenance
- Easy to deploy, backup, clone, (live) migrate

#### Security
- VM introspection (VMI)
- Antivirus out of the OS
---

## Definition ##

> The act of creating a **virtual (rather than actual) version of something**, including virtual computer hardware platforms, storage devices, and computer network resources. (wiki)

> **A layer mapping** its visible interface and resources onto the interface and resources of the underlying layer or system on which it is implemented (CMU)

> An enfficient, isolated **duplicate of the real machine** ([Formal Requirements for Virtualizable Third Generation Architectures](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.141.4815&rep=rep1&type=pdf))
- Efficiency
    - Innocuous instructions should execute directly on hardware
- Resource control
    - Executed programs may not affect the system resources
- Equivalence
    - The behavior of a program executing under the VMM should be the same as if the program were executed directly on the hardware (except possibly for timing and resource availability)

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## Basic Concepts ##

![](https://silentming.net/images/2018/20180613virtualization00.png)

* Domain/VM (Virtual Machine)

* Guest VM / Guest OS / Guest App

* Hypervisor / VMM (VM Monitor) / host

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## Type of Hypervisor ##

![](https://silentming.net/images/2018/20180613virtualization01.png)

Who manages hardware? (Type-1: Hypervisor, Type-2: Host Kernel)
* Type-1: Xen
    * Own scheduler, I/O Driver...
* Type-2: Kvm
    * Multiplex scheduler and drivers in Linux

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# Classification and Spectrum

## Classfication

- CPU Virtualization (privilege level & privileged instructions)
- Memory Virtualization (GVA -> GPA -> HPA/MPA)
- I/O Virtualization

## Spectrum

- Traditional Full Virtualization
- Para-Virtualization
- Hardware-Assistant Virtualization

---

## Approach of Virtualization ##

|Approach|Traditional Full Virtualization|Para-Virtualization|Hardware-assistant Virtualization<hr>|
|:---|:---|:---|:---|
|CPU|Binary Rewriting|Using hypercall|Root/Non-root (VT-x) host mode & guest mode (AMD-v) EL2 (ARM)<hr>|
|Memory|Software Emluation|Shadow Page Table|Extended PT (VT-x) Nested PT(AMD-v) EL2 translation table (ARM)<hr>|
|I/O|Software Emluation|Para / virt I/O (Front & Back-end)|Singe Root I/O Virtualization|

---

## Approach of Virtualization cont'd ##

#### Software Emulation

```c
/* Complete Machine Simulation */
#define REG_EAX 1;
int32_t eip;
int32_t regs[8];
int32_t segregs[4];
...
for (;;) {
   read_instruction();
   switch (decode_instruction_opcode()) {
   case OPCODE_ADD:
      int src = decode_src_reg();
      int dst = decode_dst_reg();
      regs[dst] = regs[dst] + regs[src];
      break;
  case ..
  }
  eip += instruction_length;
}
```

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# Type-2 Virtualization on ARMv8 #

|Approach|Traditional Full Virtualization|Para-Virtualization|Hardware-assistant Virtualization<hr> |
|:---|:---|:---|:---|
| CPU|Binary Rewriting|Using hypercall | Root/Non-root (VT-x) host mode & guest mode (AMD-v) **EL2 (ARM)** <hr> |
| Memory|Software Emluation | Shadow Page Table | Extended PT (VT-x) Nested PT(AMD-v) **EL2 translation table (ARM)** <hr> |
| I/O|Software Emluation | Para / **virt I/O (Front & Back-end)** | Singe Root I/O Virtualization |

---

## Run Application ##

![](https://silentming.net/images/2018/20180613virtualization02.png)

1. Load application from image to memory;
2. Give control to app;
3. App trap on privileged instructions during execution;
4. Kernel handle exception and return control;

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## Run Guest VM ##

![](https://silentming.net/images/2018/20180613virtualization03.png)

1. Load guest image into memory;
2. Give control to guest OS;
3. Control return to hypervisor when VMExit happen;
4. Hypervisor handle VMExit and return control using VMRun(AMD-V)/VMEnter(VT-x)/IRET(ARM);

---

## Challenges

1. Privilege instructions in guest APP traps into which level?

2. Privilege level of Guest OS? (Hypervisor has highest privilege)

3. Guest OS need physical memory.

4. I/O devices need to be shared by different VMs.

---

### CPU Virtualization - ARMv8 Virtualization Support

![](https://silentming.net/images/2018/20180613virtualization04.png)

* EL2: New Exception Level for Hypervisor
 - Separate CPU mode designed to run hypervisor
 - **Not** designed to run full operating system
 - Reduced virtual memory support compared to EL1
 - Limited support for interracting with EL0

---

### CPU Virtualization - ARMv8 Virtualization Support

#### Hypervisor Configuration Register (HCR\_EL2)

* **Purpose**: Provides configuration control for virtualization, including whether various Non- secure operations are trapped to EL2. HCR_EL2 is part of the Hypervisor and virtualization registers functional group.

> ARM© Cortex©-A73 MPCore Processor revision: r0p2
> Technical Reference Manual
> 4.3.43 Hypervisor Configuration Register

---

### CPU Virtualization cont'd

$LINUX/arch/arm64/kvm/hyp/entry.S
```
ENTRY(__guest_enter)
	// x0: vcpu
	// x1: host context
	// x2-x17: clobbered by macros
	// x18: guest context

// Store the host regs
    ...

// Restore guest regs x0-x17
	ldp	x0, x1,   [x18, #CPU_XREG_OFFSET(0)]
    ...
	
	// Restore guest regs x19-x29, lr
	restore_callee_saved_regs x18

// Restore guest reg x18
	ldr	x18,      [x18, #CPU_XREG_OFFSET(18)]

// Do not touch any register after this!
	eret
ENDPROC(__guest_enter)
```

---

### CPU Virtualization cont'd

$LINUX/arch/arm64/kvm/hyp/entry.S
```
ENTRY(__guest_exit)
	// x0: return code
	// x1: vcpu
	// x2-x29,lr: vcpu regs
	// vcpu x0-x1 on the stack
    ...

// Store the guest regs x2 and x3
	// Retrieve the guest regs x0-x1 from the stack
	// Store the guest regs x0-x1 and x4-x18
	// Store the guest regs x19-x29, lr

get_host_ctxt	x2, x3

// Now restore the host regs
	restore_callee_saved_regs x2

// If the exception took place, restore the EL1 exception
	// context so that we can report some information.
	msr	elr_el2, x2
	msr	esr_el2, x3
	msr	spsr_el2, x4
	orr	x0, x0, x5
1:	ret
ENDPROC(__guest_exit)
```

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### Memory Virtualization - EL2 Translation Table

![](https://silentming.net/images/2018/20180613virtualization05.png)

* An independant translation table in EL2
* GVA -> GPA -> HPA
 * VA -> IPA (Intermediate Physical Address) -> PA

---

### I/O Virtualization - virtio in QEMU

![](https://silentming.net/images/2018/20180613virtualization06.png)

* Front-driver in guest kernel;
* Back-driver in QEMU;
* Transfer data via Shared-memory (Vring);
* Qemu rw data using real driver;

---

# Thanks #