How to Contribute to crosvm

How to report bugs

We use Google issue tracker. Please use the public crosvm component.

For Googlers: See go/crosvm#filing-bugs.

Contributing code

Gerrit Account

You need to set up a user account with gerrit. Once logged in, you can obtain HTTP Credentials to set up git to upload changes.

Once set up, run ./tools/cl to install the gerrit commit message hook. This will insert a unique "Change-Id" into all commit messages so gerrit can identify changes. Even warning messages appear, the message hook will be installed.

Contributor License Agreement

Contributions to this project must be accompanied by a Contributor License Agreement (CLA). You (or your employer) retain the copyright to your contribution; this simply gives us permission to use and redistribute your contributions as part of the project. Head over to to see your current agreements on file or to sign a new one.

You generally only need to submit a CLA once, so if you've already submitted one (even if it was for a different project), you probably don't need to do it again.

Commit Messages

As for commit messages, we follow ChromeOS's guideline in general.

Here is an example of a good commit message:

devices: vhost: user: vmm: Add Connection type

This abstracts away the cross-platform differences:
cfg(any(target_os = "android", target_os = "linux")) uses a Unix
domain domain stream socket to connect to the vhost-user backend, and
cfg(windows) uses a Tube.

TEST=tools/presubmit --all

Change-Id: I47651060c2ce3a7e9f850b7ed9af8bd035f82de6
  • The first line is a subject that starts with a tag that represents which components your commit relates to. Tags are usually the name of the crate you modified such as devices: or base:. If you only modified a specific component in a crate, you can specify the path to the component as a tag like devices: vhost: user:. If your commit modified multiple crates, specify the crate where your main change exists. The subject should be no more than 50 characters, including any tags.
  • The body should consist of a motivation followed by an impact/action. The body text should be wrapped to 72 characters.
  • BUG lines are used to specify an associated issue number. If the issue is filed at Google's issue tracker, write BUG=b:<bug number>. If no issue is associated, write BUG=None. You can have multiple BUG lines.
  • TEST lines are used to describe how you tested your commit in a free form. You can have multiple TEST lines.
  • Change-Id is used to identify your change on Gerrit. It's inserted by the gerrit commit message hook as explained in the previous section. If a new commit is uploaded with the same Change-Id as an existing CL's Change-Id, gerrit will recognize the new commit as a new patchset of the existing CL.

Uploading changes

To make changes to crosvm, start your work on a new branch tracking origin/main.

git checkout -b myfeature --track origin/main

After making the necessary changes, and testing them via Presubmit Checks, you can commit and upload them:

git commit
./tools/cl upload

If you need to revise your change, you can amend the existing commit and upload again:

git commit --amend
./tools/cl upload

This will create a new version of the same change in gerrit.

If the branch contains multiple commits, each one will be uploaded as a separate review, and they will be linked in Gerrit as related changes. You may revise any commit in a branch using tools like git rebase and then re-upload the whole series with ./tools/cl upload when HEAD is pointing to the tip of the branch.

Note: We don't accept any pull requests on the GitHub mirror.

Getting Reviews

All submissions needs to be reviewed by one of the crosvm owners. Use the gerrit UI to request a review and add to assign to a random owner.

If you run into issues with reviews, reach out to the team via chat or email list.

For Googlers: see go/crosvm-chat.

Any change to Cargo.lock

When adding a new crate from, additional review is required to ensure that the crate meets the crosvm project standards. This review is provided by the members of OWNERS_COUNCIL.

Unfortunately, our tooling cannot tell the difference between adding an external crate and changing dependencies within crosvm (e.g. devices depending on a new internal crosvm utility crate). For those cases, a rubberstamp is still needed from OWNERS_COUNCIL.

For Googlers: see go/crosvm/3p_crates.

Reviewing code (for OWNERS)

We have two major types of reviewers on the project:

  1. Global OWNERS: these folks are broadly responsible for the health of the crosvm project, and have expertise in multiple project subdomains. While they can technically approve any change, they will often delegate to area OWNERS when a change is outside their expertise.
  2. Area OWNERS: experts in a particular subdomain of the project (e.g. graphics, USB, etc). Major changes in an area SHOULD be reviewed by an area OWNER, if one exists (not all subdomains have OWNERS).

All owners are expected to review code in their areas, and to aim for the following goals in reviews:

  • Reply to reviews within 1 working day. If this is infeasible (especially if overloaded), reassign to crosvm-reviews@ to pick another OWNER at random.
  • Defer to the styleguide where it makes sense to do so. Update the styleguide when it does not.
  • Strive to avoid reviews getting stuck in endless back & forth. If you see this happening, you can:
    • Schedule a meeting to discuss it online. Consider inviting another OWNER to help brainstorm solutions.
    • Bring the review discussion to the hallway chat to let the group weigh in.
  • Follow generally accepted practices for good code review
    • Technically: We insist on good documentation, clean APIs especially when broadly consumed, and generally keep code health in mind.
    • Socially: Our goal, above all else, is to be good peers to each other. So we review code, not authors. We remember to disagree respectfully, and that a code review is a team effort (author and reviewer) against a hard technical problem.

Submitting code

Crosvm uses a Commit Queue, which will run pre-submit testing on all changes before merging them into crosvm.

Once one of the crosvm owners has voted "Code-Review+2" on your change, you can use the "Submit to CQ" button, which will trigger the test process.

Gerrit will show any test failures. Refer to Building Crosvm for information on how to run the same tests locally.

Each individual change in a patch series must build and pass the tests. If you are working on a series of related changes, ensure that each incremental commit does not cause test regressions or break the build if it is merged without the later changes in the series. For example, an intermediate change must not trigger any unused code warnings or cause test failures that are fixed by later changes in the series.

When all tests pass, your change is merged into origin/main.

Contributing to the documentation

The book of crosvm is built with mdBook. Each markdown file must follow Google Markdown style guide.

To render the book locally, you need to install mdbook and mdbook-mermaid, which should be installed when you run ./tools/install-deps script. Or you can use the tools/dev_container environment.

cd docs/book/
mdbook build

Output is found at docs/book/book/html/.

To format markdown files, run ./tools/fmt in the dev_container.