Style guide

Hafnium's coding style has been based on the Linux style with explicit modifications:

  • Always use braces for conditionals and loops. (No SSL goto fail;, thanks.)

Following this, we generally fall back to the subset of the Google C++ style guide that is applicable to C.

We try to automate this where possible with clang-format and clang-tidy but that doesn‘t capture everything we’d like today. Where the style enforced by this tooling conflicts with what is in this document we accept what the tooling requires, and try to improve it if possible.


  • Yes, it does mean all variables are declared, C90-style, at the top of scope, even those loop induction variables.
  • Linux encourages no braces around single-statement branches. We follow Google and require braces around all scope blocks.

Naming symbols

  • Arch-specific functions should start with arch_.
  • Platform-specific functions should start with plat_.
  • Non-static functions should generally start with the name of the file they are declared in (after the arch_ or plat_ prefix if appropriate), though there are quite a few exceptions to this rule.
  • Prefer x_count over num_x.


These rules apply to comments and other natural language text.

  • Capitalize acronyms.

    • CPU, vCPU, VM, EL2, FF-A, QEMU
  • Spell out Hafnium in full, not Hf.

  • Use single spaces.

  • Sentences end with full stops.

  • If the comment fits on one line use /* */, otherwise space it out:

     * Informative long comment
     * with extra information.
  • Doc-ish comments start with /**.

    • Use for:
      • Function definitions (not declarations)
      • Struct declarations
      • Enum values
    • Do not use for:
      • Macros
      • Definitions of globals
  • References to code symbols use backticks, e.g. `my_symbol`.

Coding practices

  • Function macros should be functions instead, that way you get types.
  • Lock ordering is described at the top of api.c.
  • Use opaque types to avoid implicit casts when it will help avoid mistakes. e.g. addr.h
  • Avoid inline casting. C doesn't give much protection so be formal about the transformations. e.g. addr.h
  • If a function acquires a resource, there must be a single exit path to free the resource. Tracking down multiple exit points is hard and requires duplicated code which is harder. This may require splitting functions into subfunctions. Early exit is okay if there aren't any clean up tasks.
  • Don't use function pointers. It makes analysis hard and is often a target of attacks.
  • Be liberal with CHECK. Use it to assert pre-/post- conditions.
  • No self-modifying code.
  • Build targets should include all the direct dependencies for their sources, where possible, rather than relying on transitive dependencies.


Hafnium uses the same log levels as Arm Trusted Firmware. There are 5 log levels, in order of severity:

  1. ERROR

    Use this only for cases that there is an error in the hypervisor itself, perhaps caused by a coding error, bad configuration, unexpected hardware behaviour or a malformed manifest. Errors should not be logged during normal operation, even in case of a buggy or malicious VM.


    Use this sparingly for important messages which should be logged even in production builds because they will be useful for debugging. This is a suitable level to use for events which may indicate a bug in a VM.


    Use this for warnings which are important to developers but can generally be ignored in production.

  4. INFO

    Use this to provide extra information that is helpful for developers.


    Use this to provide even more information which may be helpful when tracing through execution in detail, such as when debugging test failures. This is the only level which should include any sensitive data.

Logging is done with the dlog_* macros, e.g. dlog_info. These accept printf-style format strings and arguments.

The log level of a build is controlled by the log_level argument defined in This defaults to INFO for debug builds and tests, meaning that all levels except VERBOSE will be logged. It is recommended to set the log level to NOTICE for production builds, to reduce binary size and log spam.