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LIVE VIRTUAL MACHINE MIGRATION on AMD PROCESSORS Re-Scan the Hardware Upon Resume

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LIVE VIRTUAL MACHINE MIGRATION on AMD PROCESSORS Re-Scan the Hardware Upon Resume AMD WHITE PAPER Live Migration with AMD-V™ Extended Migration Technology Virtual Machine migration is a capability being increasingly utilized in today’s enterprise environments. With live migration, a Virtual Machine Monitor (VMM) moves a running Virtual Table of Contents Machine (VM) nearly instantaneously from one server to another for a seamless experience to the end user while maintaining guest uptime. However, if the processors running in the 1. Introduction . 1 computers in the pre and post migration environment are not identical, live migration can result 2. Virtual Machine in unexpected behavior of the guest software. Since the introduction of its AMD64 processor Migration . 2 technology in 2003, AMD has worked closely with virtualization software developers to defi ne the functionality necessary to ensure that live migration is possible across a broad range of 3. Determining Processor Features . 2 AMD64 processors. This whitepaper discusses the basics of live migration and describes the functionality currently available to enable virtualization software vendors to support live 4. VMM Basics. 3 migration across a broad range of AMD64 processors. 5. Problems with Live Migration . 3 1. INTRODUCTION Virtualization can increase utilization of computing System virtualization is the ability to abstract and pool resources by consolidating the workloads running on 5.1 Backward Compatibility resources on a physical platform. This abstraction many physical systems into virtual machines running Problem . 3 decouples software from hardware and enables multiple on a single physical system. Virtual machines can be 5.2 Forward Compatibility operating system images to run concurrently on a single provisioned on-demand, replicated and migrated. Problem . 3 physical platform without interfering with each other. Virtual machine (VM) migration, which is the ability 6. AMD-V™ Extended As a technique, system virtualization has existed for to move a VM from one physical server to another Migration Technology . 3 decades on mainframes. In the past, industry standard under virtual machine monitor (VMM) control, is 6.1 Backward Compatibility x86-based machines, with their limited computing a capability being increasingly utilized in today’s Assurance . 3 resources and challenges in correctly virtualizing the enterprise environments. Instruction Set Architecture (ISA)[1,2], did not lend 6.1.1 Controlling Information themselves well to the technique. However, with the In this whitepaper we discuss how AMD-V Extended Returned via CPUID. 4 increase in computing resources of x86-based servers Migration Technology is designed to enable virtual 6.1.2 CPUID Override Model in recent years and with new processor extensions machine migration between AMD64 processors. Specifi c Registers . 4 such as those found in AMD-V™[3] technology, x86-based virtualization is seeing widespread adoption 6.2 Forward Compatibility and is rapidly becoming a pervasive and integral Assurance . 5 capability in enterprise environments[4]. 6.2.1 Feature Disable Bits . 6 7. Conclusions . 6 8. References . 6 Appendix: Using AMD-V Extended Migration on VMware ESX 3.0.1 . 6 AMD White Paper: Live Migration with AMD-V™ Extended Migration Technology FIGURE 1 – LIVE VIRTUAL MACHINE MIGRATION ON AMD PROCESSORS re-scan the hardware upon resume. Depending on their Virtual Machines Virtual Machines policies and the hardware differences between current and previous hardware, the OSes may refuse to resume and require a reboot. apps apps apps apps apps apps kernel kernel kernel kernel kernel kernel After a live migration, guest software continues to maintain an identical view of the pre and post migration hardware. In this whitepaper we discuss how AMD64 Processors provide support for a VMM to hide Hypervisor Live Migration Technology Hypervisor differences in software-visible processor features during Live and Cold Migration. For simplicity we will AMD Server AMD Server use Live Migration to refer to both migration methods. 3. DETERMINING PROCESSOR FEATURES 2. VIRTUAL MACHINE MIGRATION Software should use CPUID to determine processor There are many ways to migrate a VM. In static migration, features and use this information to select appropriate the VM is shutdown using OS-supported methods; its code paths. CPUID is an x86 instruction that can be static VM image is moved to another VMM and restarted. executed from kernel and applications. The following In cold migration, the VM is suspended using OS- example shows CPUID being called from an application. supported or VMM-supported methods. The suspended VM image is moved to a VMM on a different machine //Example 1: Console application on Windows® XP using CPUID and resumed. In live migration, the VMM moves a running instruction to determine //processor revision and feature information. VM instance nearly instantaneously from one server to another. Live Migration allows for dynamic load balancing #include "stdafx.h" of virtualized resource pools, hardware maintenance #include <stdio.h> without downtime and dynamic failover support. int _tmain (int argc, _TCHAR* argv[]) For simplicity in the following sections we call software { int version; int features_1; running in the VM “guest software.” int features_2; As long as the hardware in the pre and post migration __asm { environment is identical, guest software should behave mov eax, 1 in exactly the same way before and after the migration. It is when guest software runs in a different hardware cpuid environment after a migration that certain challenges mov version, eax can arise. mov features_1, ecx mov features_2, edx } Note that even though a VMM presents a virtual platform to guest software, there could be certain printf (“Processor version: 0x%x features_1: 0x%08x interfaces, depending on VMM design, which guest features_2: 0x%08x\n”, version, features_1, features_2); software can directly use to determine underlying return 0; hardware’s capabilities. We will discuss this more } in Section 5. However, a small number of existing commercial After the reboot/restart following a static migration, guest software determines the presence of certain software should go through its platform discovery phase processor features using non-standard ways (such and be able to adjust to any differences in underlying as using a try-catch code block to check if certain (virtual) hardware. instructions are supported) or assumes the presence of certain features without testing for them. With such Following a cold migration, guest software may software, it is diffi cult to provide assurance about their continue to maintain an identical view of the pre and behavior under live migration scenarios. Note that such post migration hardware. When suspended using software is equally likely to fail in non-virtualized OS-supported methods, some operating systems will environments with different processors. 2 AMD White Paper: Live Migration with AMD-V Extended Migration Technology 4. VMM BASICS To help ensure that guest software does not malfunction To understand the implications of VMM design on after a live migration, the virtualization infrastructure VM migration it is useful to briefl y review various VMM should address the following problems. design alternatives. 5.1 BACKWARD COMPATIBILITY PROBLEM It is important to note that for performance reasons, This is the case when the target processor does a VMM may only want to interfere in the execution not implement a feature which is implemented on of guest software when the guest code performs the current processor. privileged operations [1,2]. In such a case, after live migration, use of such a The fi rst method is called para-virtualization, where the feature may result in unexpected software behavior. guest’s kernel is modifi ed to cooperate with the VMM For example, use of an instruction not implemented on when performing privileged operations. The second the target processor will result in a #UD fault. method is called binary translation, where the VMM modifi es the guest’s binary image at runtime to get 5.2 FORWARD COMPATIBILITY PROBLEM processor control when guest software attempts to This is the case when the target processor implements perform a privileged operation [2]. The VMM can then features not supported on the current processor. emulate the privileged operation and return control to guest software. The third method is hardware-assisted This could be problematic in cases where the target virtualization where the VMM uses processor extensions processor uses an opcode (to implement an instruction) such as AMD-V to intercept and emulate privileged which is not defi ned on the current processor and operations in the guest. software on the current processor expects use of this undefi ned opcode to generate a #UD fault. In such 5. PROBLEMS WITH LIVE MIGRATION cases, after a live migration, software will observe a In the following sections we call the processor in different processor behavior, i.e. it will not observe #UD pre-migration environment the “current” processor. fault upon use of the (previously undefi ned) opcode. The one in post-migration environment is called the “target” processor. Fortunately a very small number of existing commercial software depends on such processor behavior. AMD When performing a live migration, a VM is suspended; its actively works with its software partners to discourage current (guest and VMM) state is captured, moved to use of such behaviors. another VMM, and resumed on it. The VMM preserves the state and identity of virtual devices after the 6. AMD-V EXTENDED MIGRATION TECHNOLOGY migration. This process happens quickly to present a Since the introduction of its AMD64 processor seamless experience to the end user and to maintain technology in 2003, AMD has worked closely with any guest uptime guarantees. virtualization software developers to defi ne the functionality necessary so that live migration is possible As indicated previously, when hardware in the pre and across a broad range of product. AMD-V™ Extended post migration environment is different, live migration Migration Technology provides various capabilities to can result in unexpected guest behavior. address problems associated with Live Migration.
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