By Barb Darrow
October 5, 2017

One knock on public clouds from Google, Amazon, and even Microsoft is that they are terrific for brand-new software built to take advantage of their resources, but not so great for existing, older business software developed and deployed before the cloud era kicked off 10 years ago.

And the cloud players are working to address that issue. Many Fortune 500 companies don’t necessarily want to rebuild applications that run just fine so they can move to someone else’s cloud data centers.

In this vein, Google (goog) last week announced that Google Cloud Platform now supports allow what techies call “nested virtualization.” Virtualization is technology that lets companies pack more software onto fewer servers and is a foundation for cloud computing. Google’s cloud uses KVM virtualization. This new technology will let customers take existing virtual machines—the basic units of virtualized computing—and encapsulate them in a Google virtual machine (think Russian nesting dolls). And there, the old VM will run in the new cloud, albeit with a slight performance hit.

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Google says businesses using software from startup appOrbit can move existing applications—along with associated data—to the Google cloud without having to rewrite them. From the Google blog: “Customers using appOrbit with Google’s nested virtualization can rapidly deploy and run a broader set of application workloads with zero modification, as well as simultaneously manage hybrid infrastructure and realize the full value of hybrid IT.”

Related: Google Woos Big Cloud Customers

But there are limitations, according to the Google documentation. First, the nested virtualization works only with late model Intel (intc) processors. Second, rival virtualization technologies like the ESX hypervisor that powers VMware (vmw), the Xen hypervisor underlying Amazon (amzn) Web Services, and Microsoft (msft) Hyper-V hypervisors are not supported. At least not yet. Google is working to add support for other hypervisors, sources tell Fortune.

There are workarounds to address the hypervisor issue now, but they require engineering tweaks that companies may not want to do.

The Microsoft Azure cloud already supports nested virtualization. AWS can support it with help from third-party Ravello Systems, which Oracle (orcl) bought last year.

Related: Microsoft Apologizes for Cloud Snafu

Google is not alone in its quest to help businesses move existing apps but it is the third entrant in a three-horse cloud race.

Amazon Web Services, the leader in public cloud, hopes its partnership with VMware will make businesses more comfortable moving existing virtualized applications to AWS. VMware Cloud on AWS. broadly available for a month, lets VMware customers move their vSphere applications—and there are many of them—to AWS and run them there.

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And, Microsoft, which has the deepest roots in enterprise accounts of the three, is pushing Azure Stack—which bundles Microsoft data center software with servers from HPE (hpe), Lenovo, and Dell Technologies. That hardware-software combo promises that companies can divvy applications up between corporate data centers and Microsoft Azure cloud. But even here, the emphasis is on brand new cloud applications as opposed to legacy software that has run for years.

The important takeaway here is that the cloud companies really want business customers to rewrite their applications to take full advantage of their respective network of cloud data centers. But they also know they have to make those cloud data centers more welcoming to older applications as well.

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