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.