Talking to the kernel

The system call interface: the only way from your app into the kernel.

Preview interface

The app developer kit (SDK) is not public yet. The snippets here show the shape of the interface described in the v0.1 specification. Names, numbers and signatures can change before release. For early access, write to [email protected].

A system call is how an app asks the kernel for something. DurgX has a deliberately short list: about a dozen native calls. Linux's 300+ calls are handled outside the kernel (see Linux apps).

The calls

Call	What it does	Can block?
`SYS_CAP_INVOKE`	Use a capability: the main way to act on kernel objects.	No
`SYS_IPC_CALL`	Send a quick message and wait for the reply.	Yes
`SYS_IPC_RECV`	Wait for a message to arrive.	Yes
`SYS_RING_NOTIFY`	Tell the kernel there is new work in a ring.	Never
`SYS_VM_MAP`	Map memory into your address space.	No
`SYS_YIELD`	Give up the CPU.	Reschedules

How to make a call

Register	Role
`rax`	System call number on entry. Return value on exit.
`rdi`, `rsi`, `rdx`, `r10`, `r8`, `r9`	Arguments 1 to 6. The fourth argument is in `r10`, not `rcx`.
`rcx`, `r11`	Overwritten by the CPU's `SYSCALL` instruction.
`rdi`, `rsi`, `rdx`	Treat them as overwritten after the call.
`rbx`, `rbp`, `r12` to `r15`, `r8`, `r9`, `r10`	Preserved across the call.

a reusable wrapper

use core::arch::asm;

/// Raw DurgX system call with up to six arguments.
#[inline(always)]
pub unsafe fn syscall6(nr: u64, a0: u64, a1: u64, a2: u64, a3: u64, a4: u64, a5: u64) -> u64 {
    let ret: u64;
    asm!(
        "syscall",
        inlateout("rax") nr => ret,
        inout("rdi") a0 => _,
        inout("rsi") a1 => _,
        inout("rdx") a2 => _,
        in("r10") a3,
        in("r8") a4,
        in("r9") a5,
        lateout("rcx") _,
        lateout("r11") _,
        options(nostack),
    );
    ret
}

Errors

Failures come back as negative numbers in rax, in the same style as Linux's -errno. Unknown calls return an invalid-syscall error. Permission problems are reported as capability faults. See Permissions.

Note

Numeric call values and exact argument layouts per call are published in the SDK headers.