Microsoft and Amazon.com have reached an agreement that gives each company a license to the other's patent portfolio, in a way that could revive one of the Redmond company's longest-running controversies. That's because, as part of the deal, Microsoft says it's granting Amazon patent-related "coverage" for its use of open-source and proprietary technologies in its Kindle e-reader, and its use of Linux-based computer servers.

One practical effect of such cross-licensing deals, as they're known, is to head off potential patent disputes. In that way, the agreement amounts to a peace accord between the companies. At the same time, the deal could raise eyebrows in some corners of the technology world, if it's interpreted as Amazon implicitly endorsing Microsoft's claims that Linux and other open-source technologies violate its patents. Open-source advocates have long disputed those claims.

Microsoft says in a news release that Amazon.com will pay "an undisclosed amount of money under the agreement" -- signaling that Amazon is paying to avoid a possible patent claim from Microsoft. No terms of the deal were disclosed.

The agreement is further evidence that the Seattle-area corporate giants are increasingly crossing paths, and vying for some of the same markets, particularly as Amazon.com expands beyond its roots as an online retailer into new parts of the technology industry.

Microsoft roiled the open-source community in 2007 when it claimed that Linux and other open-source technologies violated more than 200 of its patents. The company sued TomTom last year over the navigation company's implementation of the Linux kernel in its devices. TomTom ultimately settled with Microsoft in an agreement that required it to remove Linux-related file-management technologies from its products.

The news release about the Amazon patent agreement was issued by Microsoft this afternoon, and doesn't include a statement from Amazon. We're reaching out to the online retailer for comment. [Update: Amazon declined to comment beyond what was said by Microsoft in the news release.]

Under such cross-licensing deals, companies generally agree not to sue one another over alleged patent violations in their respective products. A Microsoft representative declined to say which of its products are covered by the deal, citing the fact that the terms of the agreement are confidential (despite the fact that Microsoft disclosed in its news release the Amazon products covered by the deal).

[Follow-up: Linux boosters shrug off Amazon's Microsoft patent deal]

Microsoft says it has reached more than 600 agreements since launching its intellectual-property licensing initiative in 2003. Some of those deals have involved other companies that use Linux in their products, including LG Electronics, Samsung Electronics, Fuji Xerox, and Brother.

“We are pleased to have entered into this patent license agreement with Amazon.com,” said Horacio Gutierrez, corporate vice president and deputy general counsel, in the news release. “Microsoft’s patent portfolio is the largest and strongest in the software industry, and this agreement demonstrates our mutual respect for intellectual property as well as our ability to reach pragmatic solutions to IP issues regardless of whether proprietary or open source software is involved.”

We've asked whether Microsoft and Amazon will actively review each other's patent portfolios in search of new technologies to incorporate into their products, as a result of the deal, and we'll update this post depending on the response.

Previously: Microsoft and Amazon square off, sorta, in a very Seattle rivalry

Post updated at 6:55 p.m. with additional details, context.