Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

  • Print
  • + Share This
This chapter is from the book

This chapter is from the book

A Bit of History

I started working on UML in earnest in February 1999 after having the idea that porting Linux to itself might be practical. I tossed the idea around in the back of my head for a few months in late 1998 and early 1999. I was thinking about what facilities it would need from the host and whether the system call interface provided by Linux was rich enough to provide those facilities. Ultimately, I decided it probably was, and in the cases where I wasn't sure, I could think of workarounds.

So, around February, I pulled a copy of the 2.0.32 kernel tree off of a Linux CD (probably a Red Hat source CD) because it was too painful to try to download it through my dialup. Within the resulting kernel tree, I created the directories my new port was going to need without putting any files in them. This is the absolute minimum amount of infrastructure you need for a new port. With the directories present, the kernel build process can descend into them and try to build what's there.

Needless to say, with nothing in those directories, the build didn't even start to work. I needed to add the necessary build infrastructure, such as Makefiles. So, I added the minimal set of things needed to get the kernel build to continue and looked at what failed next. Missing were a number of header files used by the generic (hardware-independent) portions of the kernel that the port needs to provide. I created them as empty files, so that the #include preprocessor directives would at least succeed, and proceeded onward.

At this point, the kernel build started complaining about missing macros, variables, and functions—the things that should have been present in my empty header files and nonexistent C source files. This told me what I needed to think about implementing. I did so in the same way as before: For the most part, I implemented the functions as stubs that didn't do anything except print an error message. I also started adding real headers, mostly by copying the x86 headers into my include directory and removing the things that had no chance of compiling.

After defining many of these useless procedures, I got the UML build to "succeed." It succeeded in the sense that it produced a program I could run. However, running it caused immediate failures due to the large number of procedures I defined that didn't do what they were supposed to—they did nothing at all except print errors. The utility of these errors is that they told me in what order I had to implement these things for real.

So, for the most part, I plodded along, implementing whatever function printed its name first, making small increments of progress through the boot process with each addition. In some cases, I needed to implement a subsystem, resulting in a related set of functions.

Implementation continued in this vein for a few months, interrupted by about a month of real, paying work. In early June, I got UML to boot a small filesystem up to a login prompt, at which point I could log in and run commands. This may sound impressive, but UML was still bug-ridden and full of design mistakes. These would be rooted out later, but at the time, UML was not much more than a proof of concept.

Because of design decisions made earlier, such fundamental things as shared libraries and the ability to log in on the main console didn't work. I worked around the first problem by compiling a minimal set of tools statically, so they didn't need shared libraries. This minimal set of tools was what I populated my first UML filesystem with. At the time of my announcement, I made this filesystem available for download since it was the only way anyone else was going to get UML to boot.

Because of another design decision, UML, in effect, put itself in the background, making it impossible for it to accept input from the terminal. This became a problem when you tried to log in. I worked around this by writing what amounted to a serial line driver, allowing me to attach to a virtual serial line on which I could log in.

These are two of the most glaring examples of what didn't work at that point. The full list was much longer and included other things such as signal delivery and process preemption. They didn't prevent UML from working convincingly, even though they were fairly fundamental problems, and they would get fixed later.

At the time, Linus was just starting the 2.3 development kernel series. My first "UML-ized" kernel was 2.0.32, which, even at the time, was fairly old. So, I bit the bullet and downloaded a "modern" kernel, which was 2.3.5 or so. This started the process, which continues to this day, of keeping in close touch with the current development kernels (and as of 2.4.0, the stable ones as well).

Development continued, with bugs being fixed, design mistakes rectified (and large pieces of code rewritten from scratch), and drivers and filesystems added. UML spent a longer than usual amount of time being developed out of pool, that is, not integrated into the mainline Linus' kernel tree. In part, this was due to laziness. I was comfortable with the development methodology I had fallen into and didn't see much point in changing it.

However, pressure mounted from various sources to get UML into the main kernel tree. Many people wanted to be able to build UML from the kernel tree they downloaded from http://www.kernel.org. or got with their distribution. Others, wanting the best for the UML project, saw inclusion in Linus' kernel as being a way of getting some public recognition or as a stamp of approval from Linus, thus attracting more users to UML. More pragmatically, some people, who were largely developers, noted that inclusion in the official kernel would cause updates and bug fixes to happen in UML "automatically." This would happen as someone made a pass over the kernel sources, for example, to change an interface or fix a family of bugs, and would cover UML as part of that pass. This would save me the effort of looking through the patch representing a new kernel release, finding those changes, figuring out the equivalent changes needed in UML, and making them. This had become my habit over the roughly four years of UML development before it was merged by Linus. It had become a routine part of UML development, so I didn't begrudge the time it took, but there is no denying that it did take time that would have been better spent on other things.

So, roughly in the spring of 2002, I started sending updated UML patches to Linus, requesting that they be merged. These were ignored for some months, and I was starting to feel a bit discouraged, when out of the blue, he merged my 2.5.34 patch on September 12, 2002. I had sent the patch earlier to Linus as well as the kernel mailing list and one of my own UML lists, as usual, and had not thought about it further. That day, I was idling on an Internet Relay Chat (IRC) channel where a good number of the kernel developers hang around and talk. Suddenly, Arnaldo Carvalho de Melo (a kernel contributor from Brazil and the CTO of Conectiva, the largest Linux distribution in South America) noticed that Linus had merged my patch into his tree.

The response to this from the other kernel hackers, and a little later, from the UML community and wider Linux community, was gratifying positive. A surprisingly (to me) large number of people were genuinely happy that UML had been merged, and, in doing so, got the recognition they thought it deserved.

At this writing, it is three years later, and UML is still under very active development. There have been ups and downs. Some months after UML was merged, I started finding it hard to get Linus to accept updated patches. After a number of ignored patches, I started maintaining UML out of tree again, with the effect that the in-tree version of UML started to bit-rot. It stopped compiling because no one was keeping it up to date with changes to internal kernel interfaces, and of course bugs stopped being fixed because my fixes weren't being merged by Linus.

Late in 2004, an energetic young Italian hacker named Paolo Giarrusso got Andrew Morton, Linus' second-in-command, to include UML in his tree. The so-called "-mm" tree is a sort of purgatory for kernel patches. Andrew merges patches that may or may not be suitable for Linus' kernel in order to give them some wider exposure and see if they are suitable. Andrew took patches representing the current UML at the time from Paolo, and I followed that up with some more patches. Presently, Andrew forwarded those patches, along with many others, to Linus, who included them in his tree. All of a sudden, UML was up to date in the official kernel tree, and I had a reliable conduit for UML updates.

I fed a steady stream of patches through this conduit, and by the time of the 2.6.9 release, you could build a working UML from the official tree, and it was reasonably up to date.

Throughout this period, I had been working on UML on a volunteer basis. I took enough contracting work to keep the bills paid and the cats fed. Primarily, this was spending a day a week at the Institute for Security Technology Studies at Dartmouth College, in northern New Hampshire, about an hour from my house. This changed around May and June of 2004, when, nearly simultaneously, I got job offers from Red Hat and Intel. Both were very generous, offering to have me spend my time on UML, with no requirements to move. I ultimately accepted Intel's offer and have been an Intel employee in the Linux OS group since.

Coincidentally, the job offers came on the fifth anniversary of UML's first public announcement. So, in five years, UML went from nothing to a fully supported part of the official Linux kernel.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020