Crate static_keys

static-keys

Static key is a mechanism used by Linux kernel to speed up checks of seldomly used features. We brought it to Rust userland applications!

Example

A common example is about some flags user passed from commandline options to a long-running CLI applications. It is annoying to check a value that will never be changed after application boot up:

let flag = CommandlineArgs::parse();
loop {
    if flag {
        do_something();
    }
    do_common_routines();
}

Although the if-check is just test-jnz instructions in x86, it can still be speedup. What about making the check just as a jmp (skip over the do_something branch) or nop (always do_something)? This is what static keys do. To put it simply, we modify the instruction at runtime. After getting the flag passed from commandline, we dynamically modify the if flag instruction to be a jmp or nop according to the flag value.

Usage

See Example.

Modules

• code_manipulate
  Utilities to manipulate memory protection.

Macros

• define_static_key_false
  Define a static key with false value.
• static_key_likely
  Use this in a if condition, just like the usual likely and unlikely intrinsics
• static_key_unlikely
  Use this in a if condition, just like the usual likely and unlikely intrinsics

Structs

• NoStdStaticKey
  Static key to hold data about current status and which jump entries are associated with this key

Type Aliases

• StaticKey
  A convenient alias for NoStdStaticKey, utilizing nix for memory protection manipulation.