add runbook for updating the Rust version in CI

Signed-off-by: Andreea Florescu <[email protected]>

docs/readme.md

@@ -6,4 +6,5 @@ of development in the rust-vmm project.
 ## Table of Contents
 - [Code Coverage](code_coverage.md)
 - [Crate Release](crate_release.md)
-- [Set Up a New Repository](setup_new_repo.md)
+- [Set Up a New Repository](setup_new_repo.md)
+- [Update the Rust Toolchain](update-rust-toolchain.md)

docs/update-rust-toolchain.md

@@ -0,0 +1,29 @@
+# Update Rust Toolchain
+
+The Rust version used for running the tests in rust-vmm crates is fixed such
+that toolchain updates can only happen when the version is explicitly updated.
+
+The Rust version is specified in the
+[rust-vmm-container](https://github.com/rust-vmm/rust-vmm-container), and the
+rust-vmm-container version is specified in
+[rust-vmm-ci](https://github.com/rust-vmm/rust-vmm-ci). While rust-vmm-ci
+does not declare a version itself, in the rust-vmm crates we are specifying
+which commit from rust-vmm-ci to use for running the CI.
+
+Thus, to update the Rust toolchain (or for that matter any other tools in the
+CI), you need to go through the following updates:
+
+1. Update the Rust toolchain version in the rust-vmm-container
+[Dockerfile](https://github.com/rust-vmm/rust-vmm-container/blob/main/Dockerfile).
+Open a PR with your change. Once the PR is approved and merged, the merge
+will automatically trigger the publishing of a new container version. For more
+details about how this works, you can check the rust-vmm-container
+[README](https://github.com/rust-vmm/rust-vmm-container).
+2. Update the rust-vmm-container version in the rust-vmm-ci
+[autogenerate script](https://github.com/rust-vmm/rust-vmm-ci/blob/c2f8c93e3796d8b3ea7dc339fad211457be9c238/.buildkite/autogenerate_pipeline.py#L63).
+and open a PR with your change. Once this is merged, you can go ahead with
+updating the rust-vmm-ci submodule.
+3. Update the rust-vmm-ci for the target rust-vmm repository. NOTE: the
+rust-vmm-ci submodule is automatically updated using Dependabot
+every week. If for any reason you need this update faster than the default
+schedule, you can follow the runbook for manually updating rust-vmm-ci.