Amazon Web Services used to be the public cloud of choice for scrappy cash-strapped startups. Small companies liked that they could buy (or rent) computer servers, networking, and storage as needed, typically with a credit card.

Tiny companies are unlikely to spend much on VMware vmw data center software or Workday wday human resource and financial applications. And yet at this week’s AWS Re:Invent tech conference in Las Vegas, VMware CEO Pat Gelsinger and Workday CEO Aneel Bhusri will share the stage with AWS CEO Andy Jassy.

That shows that not only does Amazon amzn want to sell its data center servers and storage to big customers, but to the software providers that also sell to those customers. It wants to supply the suppliers as well as the suppliers’ customers. Towards that end, Amazon is well into year six of a push for big business customers. There have been signs of traction already: General Electric, an AWS customer, has been a presence at this conference for a few years now, and companies don’t get much bigger than that.

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Just a few weeks ago, AWS and VMware, once bitter rivals, agreed to work together to make VMware vSphere available on AWS so that joint customers could take the same software packages that now run on their own servers over to the Amazon cloud. Last spring, Salesforce crm , which sells customer management and sales software to big companies, committed to spending $400 million over four years to host new software on AWS rather than in its own data centers.

That traction results from a concerted effort Amazon management over the past four or five years to convince businesses in the financial services, healthcare, and government that running on shared public cloud infrastructure meets their security and other requirements.

And now the battle is on as AWS, Microsoft, IBM, Google fight to win big business accounts (and the software vendors selling into those accounts) for their respective clouds.

Some examples:

Microsoft msft recently signed up SAP sap SuccessFactors for its Microsoft Azure cloud. Meanwhile, AWS bragged about landing Salesforce as a big customer earlier this year. IBM, Amazon, and Hewlett Packard (before last year’s Hewlett-Packard Enterprise hpe split from Hewlett-Packard Inc. publicized that Workday used its cloud.

As an interesting aside, many of these deals smack of mutual back scratching. Does Salesforce’s commitment to spending $400 million on AWS infrastructure over the next four years have anything to do with Amazon signing a deal to give its employees access to all of Salesforce’s products?

For more on AWS, watch:

The Microsoft-SAP deal has a similar back story. In October, Microsoft proclaimed that SAP’s SuccessFactors human resources software would be available on its Azure as well as SAP’s own infrastructure. Less than a month later, SAP announced that Microsoft would use SuccessFactors for its human resources needs.

As the cloud vendors scrap over customers, expect more such deals between major software suppliers. AWS, Microsoft Azure, Google googl Cloud Platform, and IBM ibm SoftLayer all would love to be a one-stop shop for the software applications and infrastructure for big companies.