virtio-mmioとvirtqueueを自前で実装する——virtio-consoleで動作確認する

fn mmio_read(&self, offset: u32) -> u32 {
    match offset {
        0x000 => 0x7472_6976, // "virt"
        0x004 => 2,           // version (non-legacy)
        0x008 => VIRTIO_ID_CONSOLE,
        0x00c => 0x414b_4759, // vendor id (適当な自前の値)
        0x010 => {
            if self.device_features_sel == 1 {
                (VIRTIO_F_VERSION_1 >> 32) as u32
            } else {
                0
            }
        }
        0x034 => QUEUE_NUM_MAX,
        0x044 => self.queues[self.queue_sel as usize].ready as u32,
        0x060 => self.interrupt_status,
        0x070 => self.status,
        0x0fc => 0, // config generation
        _ => 0,
    }
}

fn handle_notify(&mut self, queue_index: u32, guest_memory: &GuestMemoryMmap<()>, vm: &VmFd) {
    // queue 1 = TX(ゲスト→ホスト)。queue 0(RX)は今回はセットアップのみで扱わない。
    if queue_index != 1 {
        return;
    }
    let vq = &mut self.queues[1];
    if !vq.ready || vq.num == 0 {
        return;
    }

    let avail_idx: u16 = guest_memory
        .read_obj(GuestAddress(vq.avail_addr + 2))
        .unwrap();

    while vq.last_avail_idx != avail_idx {
        let slot = (vq.last_avail_idx % vq.num as u16) as u64;
        let head: u16 = guest_memory
            .read_obj(GuestAddress(vq.avail_addr + 4 + slot * 2))
            .unwrap();

        let mut desc_index = head;
        loop {
            let entry_addr = vq.desc_addr + (desc_index as u64) * VIRTQ_DESC_SIZE;
            let addr: u64 = guest_memory.read_obj(GuestAddress(entry_addr)).unwrap();
            let len: u32 = guest_memory.read_obj(GuestAddress(entry_addr + 8)).unwrap();
            let flags: u16 = guest_memory
                .read_obj(GuestAddress(entry_addr + 12))
                .unwrap();
            let next: u16 = guest_memory
                .read_obj(GuestAddress(entry_addr + 14))
                .unwrap();

            let mut buf = vec![0u8; len as usize];
            guest_memory.read_slice(&mut buf, GuestAddress(addr)).unwrap();
            std::io::stdout().write_all(&buf).unwrap();
            std::io::stdout().flush().unwrap();

            if flags & VIRTQ_DESC_F_NEXT == 0 {
                break;
            }
            desc_index = next;
        }

        let used_slot = (vq.used_idx % vq.num as u16) as u64;
        let elem_addr = vq.used_addr + 4 + used_slot * VIRTQ_USED_ELEM_SIZE;
        guest_memory
            .write_obj(head as u32, GuestAddress(elem_addr))
            .unwrap();
        guest_memory.write_obj(0u32, GuestAddress(elem_addr + 4)).unwrap();
        vq.used_idx = vq.used_idx.wrapping_add(1);
        guest_memory
            .write_obj(vq.used_idx, GuestAddress(vq.used_addr + 2))
            .unwrap();

        vq.last_avail_idx = vq.last_avail_idx.wrapping_add(1);
    }

    self.interrupt_status |= 0x1; // VIRTIO_MMIO_INT_VRING
    let _ = vm.set_irq_line(VIRTIO_IRQ, true);
    let _ = vm.set_irq_line(VIRTIO_IRQ, false);
}

virtio_mmio.device=0x200@0xd0000000:5

[    0.241329] virtio-mmio: Registering device virtio-mmio.0 at 0xd0000000-0xd00001ff, IRQ 5.

match std::fs::OpenOptions::new().write(true).open("/dev/hvc0") {
    Ok(mut f) => {
        let _ = f.write_all(b"hello via virtio-console\n");
        let _ = f.flush();
    }
    Err(e) => {
        println!("open /dev/hvc0 failed: {e}");
    }
}

unsafe {
    let target = CString::new("/dev").unwrap();
    let fstype = CString::new("devtmpfs").unwrap();
    libc::mount(std::ptr::null(), target.as_ptr(), fstype.as_ptr(), 0, std::ptr::null());
}

[    0.446100] Run /init as init process
hello via virtio-console
[    0.446916] reboot: Restarting system