1# Style guide 2 3Hafnium's coding style has been based on the 4[Linux style](https://www.kernel.org/doc/html/v4.17/process/coding-style.html) 5with explicit modifications: 6 7* Always use braces for conditionals and loops. (No SSL `goto fail;`, thanks.) 8 9Following this, we generally fall back to the subset of the 10[Google C++ style guide](https://google.github.io/styleguide/cppguide.html) that 11is applicable to C. 12 13We try to automate this where possible with clang-format and clang-tidy but that 14doesn't capture everything we'd like today. Where the style enforced by this 15tooling conflicts with what is in this document we accept what the tooling 16requires, and try to improve it if possible. 17 18[TOC] 19 20## Clarifications 21 22* Yes, it does mean all variables are declared, C90-style, at the top of 23 scope, even those loop induction variables. 24* Linux encourages no braces around single-statement branches. We follow 25 Google and require braces around all scope blocks. 26 27## Naming symbols 28 29* Arch-specific functions should start with `arch_`. 30* Platform-specific functions should start with `plat_`. 31* Non-static functions should generally start with the name of the file they 32 are declared in (after the `arch_` or `plat_` prefix if appropriate), though 33 there are quite a few exceptions to this rule. 34* Prefer `x_count` over `num_x`. 35 36## Prose 37 38These rules apply to comments and other natural language text. 39 40* Capitalize acronyms. 41 * CPU, vCPU, VM, EL2, FF-A, QEMU 42* Spell out Hafnium in full, not Hf. 43* Use single spaces. 44* Sentences end with full stops. 45* If the comment fits on one line use `/* */`, otherwise space it out: 46 47 ``` 48 /* 49 * Informative long comment 50 * with extra information. 51 */ 52 ``` 53 54* Doc-ish comments start with `/**`. 55 56 * Use for: 57 * Function definitions (not declarations) 58 * Struct declarations 59 * Enum values 60 * Do not use for: 61 * Macros 62 * Definitions of globals 63 64* References to code symbols use backticks, e.g. `` `my_symbol` ``. 65 66## Coding practices 67 68* Function macros should be functions instead, that way you get types. 69* Lock ordering is described at the top of [`api.c`](../src/api.c). 70* Use opaque types to avoid implicit casts when it will help avoid mistakes. 71 e.g. [`addr.h`](../inc/hf/addr.h) 72* Avoid inline casting. C doesn't give much protection so be formal about the 73 transformations. e.g. [`addr.h`](../inc/hf/addr.h) 74* If a function acquires a resource, there must be a single exit path to free 75 the resource. Tracking down multiple exit points is hard and requires 76 duplicated code which is harder. This may require splitting functions into 77 subfunctions. Early exit is okay if there aren't any clean up tasks. 78* Don't use function pointers. It makes analysis hard and is often a target of 79 attacks. 80* Be liberal with `CHECK`. Use it to assert pre-/post- conditions. 81* No self-modifying code. 82* Build targets should include all the direct dependencies for their sources, 83 where possible, rather than relying on transitive dependencies. 84 85## Logging 86 87Hafnium uses the same log levels as Arm Trusted Firmware. There are 5 log 88levels, in order of severity: 89 901. `ERROR` 91 92 Use this only for cases that there is an error in the hypervisor itself, 93 perhaps caused by a coding error, bad configuration, unexpected hardware 94 behaviour or a malformed manifest. Errors should not be logged during normal 95 operation, even in case of a buggy or malicious VM. 96 972. `NOTICE` 98 99 Use this sparingly for important messages which should be logged even in 100 production builds because they will be useful for debugging. This is a 101 suitable level to use for events which may indicate a bug in a VM. 102 1033. `WARNING` 104 105 Use this for warnings which are important to developers but can generally be 106 ignored in production. 107 1084. `INFO` 109 110 Use this to provide extra information that is helpful for developers. 111 1125. `VERBOSE` 113 114 Use this to provide even more information which may be helpful when tracing 115 through execution in detail, such as when debugging test failures. This is 116 the only level which should include any sensitive data. 117 118Logging is done with the `dlog_*` macros, e.g. `dlog_info`. These accept 119printf-style format strings and arguments. 120 121The log level of a build is controlled by the `log_level` argument defined in 122[`BUILDCONFIG.gn`](../build/BUILDCONFIG.gn). This defaults to `INFO` for debug 123builds and tests, meaning that all levels except `VERBOSE` will be logged. It is 124recommended to set the log level to `NOTICE` for production builds, to reduce 125binary size and log spam. 126