How Intel Flex Migration technology helps the live migration among defferent generation of Intel ISA?

Intel Flex Migration sits between two important and evolving computing capabilities: On the one hand, Intel has had a long tradition of adding new features to our Instruction Set Architecture (ISA) over the years, such as MMX and several generations of SSE. These new ISA features accelerate a broad range of computing tasks, such as media and image processing, security functions such as encryption, and network processing. At the same time, virtual machine monitor (VMM) vendors like VMware are providing the powerful new capability of being able to migrate a virtual machine from one physical platform to another, which is very useful for datacenter load balancing and resource management.

 

To see the tension between these two technology trends, consider the following: Suppose you have a datacenter populated with a number of different servers that have been deployed over time and that therefore have different processor generations with different sets of ISA features. For example, a subset of the servers might have processors with the SSE3 instruction set, while another subset might have the more recent SSE4 feature set. Suppose that you initially start a VM on one of the SSE4-based systems. Software running in that VM will see the presence of the new SSE4 instructions and may begin to use them. Now suppose that at a later point in time you want to "live migrate" that running VM to another system in the datacenter to balance load. If that VM happens to be migrated to one of the SSE3-based platforms, then from the perspective of software running in the VM, the SSE4 instructions will suddenly vanish, causing the VM to crash. There are some ways to try to avoid these problems: you can partition your datacenter into "islands" of systems such that each share an identical feature set, and then limit VM migration only within those islands, but not between them. But with the regular introduction of new ISA features with each Intel processor generation, this approach can quickly become unmanageable.

 

There is another approach to solving the problem and it builds on the way that Intel reports the availability of new ISA features via an instruction known as "CPUID". CPUID enables software to "discover" the exact set of new features provided by any given processor implementation. For example, the CPUID instruction returns a set of bits that indicate whether SSE3, SSE4 or any other new ISA features are provided by the underlying processor. If a VMM has a way to intercept executions of CPUID by guest software running in a VM, then it could "spoof" the bits returned to trick guest software into seeing a different, consistent feature set. For example, it could make all of the systems in a datacenter look like they support the SSE3 instruction set (even if some of the systems actually support SSE4). VMs could then migrate freely between all systems in the datacenter without failures.

 

This is where Intel Flex Migration enters the picture: Prior to Flex Migration, there was no simple way for VMM software to control the values returned to guest software by the CPUID instruction. But with Flex Migration, Intel offers a couple of new mechanisms for modifying the CPUID bits under VMM control -- either through a bitmask that limits the reporting of new features, or by giving a VMM a way to intercept the entire instruction and emulate alternative return values.

 

Clearly there is a tradeoff here: To achieve the ability to migrate VMs between different physical platforms, an IT administrator must choose a "least common denominator" set of support, which may mean giving up some of the newer ISA extensions. But as older systems are retired from the datacenter, it is always possible to upgrade to newer processor features in a coordinated way as the majority of systems in a datacenter support more up-to-date ISA extensions. The key is that Intel Flex Migration -- when used together with software from VMM vendors such as VMware -- enables IT administrators to manage these transitions and to balance the relative benefits of VM migration and new ISA features on a timeline that matches their organizations requirements.