x86_64 JIT

Relevant source files

• src/jit.rs 
• src/lib.rs 
• tests/ubpf_jit_x86_64.rs 
• tests/jit_sizing.rs 

The handwritten x86_64 JIT comes from uBPF’s JIT algorithm. The jit module, jit_compile(), execute_program_jit() and set_jit_exec_memory() are compiled only under:

#[cfg(all(target_arch = "x86_64", not(windows)))]

The fork introduced this gate; the old one was not(windows). It keeps AArch64 and other architectures from building or running the x86 emitter.

Generated function signature

type MachineCode = unsafe fn(*mut u8, usize, *mut u8, usize, usize, usize) -> u64;
//                           mbuff,   mbuff_len, mem, mem_len, mem_offset, mem_end_offset

The VM wrappers call it as follows:

An empty mem is passed as a null pointer, which matches uBPF.

Register mapping

R10 holds the mem pointer for LD_ABS/LD_IND. R11 and RCX are scratch, and RCX is also used for shifts and division.

Compilation pipeline

JitMemory::new compiles twice. The first pass is a dry run: it counts bytes and writes nothing, and the result sets the buffer size. The second pass emits the code and patches relative jumps (resolve_jumps). tests/jit_sizing.rs checks that a dry run still resolves jumps correctly.

no_std executable memory

let mut vm = rbpf::EbpfVmRaw::new(Some(prog)).unwrap();
// `exec` must be page-aligned (4096), large enough for the page-rounded code,
// and already mapped executable by the caller.
vm.set_jit_exec_memory(exec)?;
vm.jit_compile()?; // consumes the buffer (Option::take)

jit_compile() returns an error when no buffer was set, when it is too small (“Executable memory is too small”), or when it is misaligned (“Executable memory is not aligned”). The buffer is taken by the compile call, so call set_jit_exec_memory again before every recompilation.

Semantics notes

• Helpers: CALL src=0 emits a direct call to the helper’s address, so the helper must be registered before jit_compile(). You can replace a helper afterwards only by recompiling. If the ID is not registered at compile time, compilation fails with “[JIT] Error: unknown helper function”.
• Local calls: CALL src=1 is supported through emit_local_call.
• Tail calls: TAIL_CALL hits unimplemented!() (panic).
• Division: division by zero gives 0. Modulo by zero leaves dst unchanged.
• Stack: STACK_SIZE (512) bytes are reserved below RBP on the native stack.

Conformance status

CI runs the bpf_conformance suite against the JIT through examples/rbpf_plugin.rs --jit, but the step is non-blocking (continue-on-error) because of upstream issue #60 . The interpreter and Cranelift conformance steps are blocking. See Testing and CI.