IPC

This chapter covers making and handling protected procedure calls in protection domains written in Rust. Navigate to and run the example:

cd workspaces/microkit/ipc
make simulate

The example system description specifies two protection domains, with a channel between them:

ipc.system:7:20
<system>
    <protection_domain name="client" priority="100">
        <program_image path="microkit-ipc-client.elf" />
    </protection_domain>

    <protection_domain name="server" priority="200" pp="true">
        <program_image path="microkit-ipc-server.elf" />
    </protection_domain>

    <channel>
        <end pd="client" id="13" />
        <end pd="server" id="37" />
    </channel>
</system>


pds/client/src/main.rs:12:32
const SERVER: Channel = Channel::new(13);

#[protection_domain]
fn init() -> impl Handler {
    debug_println!("client: initializing");
    HandlerImpl
}

struct HandlerImpl;

impl Handler for HandlerImpl {
    type Error = Infallible;

    fn notified(&mut self, channel: Channel) -> Result<(), Self::Error> {
        debug_println!("client: notified by {:?}", channel);

        debug_println!("client: TEST_PASS");

        Ok(())
    }
}

pds/server/src/main.rs:12:39
const CLIENT: Channel = Channel::new(37);

#[protection_domain]
fn init() -> impl Handler {
    debug_println!("server: initializing");

    debug_println!("server: notifying client");

    CLIENT.notify();

    HandlerImpl
}

struct HandlerImpl;

impl Handler for HandlerImpl {
    type Error = Infallible;

    fn protected(
        &mut self,
        channel: Channel,
        msg_info: MessageInfo,
    ) -> Result<MessageInfo, Self::Error> {
        debug_println!("server: called by {:?}", channel);

        todo!()
    }
}

The Channel type is the Rust equivalent of the microkit_channel type alias in libmicrokit. Note how the functionality corresponding to libmicrokit's microkit_notify, microkit_irq_ack, and microkit_ppcall is implemented in methods for Channel.

The MessageInfo type is the Rust equivalent of the microkit_msginfo type alias in libmicrokit. Just as microkit_msginfo is an alias for seL4_MessageInfo_t, sel4_microkit::MessageInfo is just a thin wrapper around MessageInfo.

libmicrokit has microkit_mr_set() and microkit_mr_get() for interacting with the IPC buffer. In the sel4_microkit crate, we have get_mr() and set_mr() , but we also have with_msg_regs(), with_msg_regs_mut(), with_msg_bytes(), and with_msg_bytes_mut(), which use sel4::with_ipc_buffer() and sel4::with_ipc_buffer_mut() under the hood.

Step 10.A (exercise)       

Exercise: In the client's notified() handler, make a protected procedure call to the server using SERVER.pp_call(). Handle the call in the server's protected() handler. Include data in the message using sel4_microkit::with_msg_regs{,_mut}.

Exercise (optional): Send something more interesting over IPC using msg_bytes and msg_bytes_mut . For example, the zerocopy crate can be used to view certain types as bytes and vice versa, and the lightweight postcard crate can be used to serialize and deserialize a wider range of types using serde.