SchOS/kernel/net/net.c

// SPDX-License-Identifier: Apache-2.0
//
// The network thread, and the one place the TCP/IP stack is started.
//
// lwIP with NO_SYS=1 has no threads of its own, so something has to feed it
// received frames and run its timers. That something is a kernel thread rather
// than the e1000 interrupt handler, and the reason is the lock: a call into
// lwIP can reach the driver, the driver can sleep, and an interrupt handler
// cannot. So the interrupt keeps doing what it always did -- clear the cause
// register -- and this thread polls.
//
// Polling costs a wakeup every few milliseconds when the link is idle. The
// alternative, waking this thread from the interrupt, is a worthwhile change
// later; it is deliberately not the first version, because a receive path that
// is woken from an interrupt is much harder to debug than one that is asked.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "proc.h"

#include "lwip/opt.h"
#include "lwip/init.h"
#include "lwip/timeouts.h"
#include "lwip/netif.h"
#include "lwip/dhcp.h"
#include "lwip/ip4_addr.h"

// Wait until the next timer tick. `ticks` advances at 100 Hz, so this is a
// sleep of up to 10 ms -- and, more importantly, it is the kernel's only timed
// sleep: there is no higher-resolution timer to wait on. The lwIP lock must not
// be held across it, or nothing else could touch the stack while the link is
// idle, which is most of the time.
static void
wait_tick(void)
{
    acquire(&tickslock);
    uint start = ticks;
    while (ticks == start)
        sleep(&ticks, &tickslock);
    release(&tickslock);
}

static int net_started;

// Set once DHCP has bound, so `ifconfig`-ish output and /proc/net have
// something true to say rather than reporting 0.0.0.0 forever.
static int net_bound;

int
net_is_up(void)
{
    return net_bound;
}

static void
net_thread(void)
{
    // lwIP's own initialisation has to happen in this thread, not in main():
    // it allocates from lwIP's pools and registers timers, and doing that
    // before there is a process context means doing it somewhere that cannot
    // sleep if the allocator ever needs to.
    lwip_sys_init();

    lwip_sys_lock();
    lwip_init();
    if (net_e1000_attach() != 0) {
        lwip_sys_unlock();
        cprintf("net: no e1000; the network thread has nothing to drive\n");
        // Not a panic and not an exit: a kernel thread must never return, and
        // a machine with no NIC is an ordinary machine.
        for (;;)
            wait_tick();
    }
    struct netif *nif = net_e1000_netif();
    dhcp_start(nif);
    lwip_sys_unlock();

    cprintf("net: lwIP %d.%d.%d up, requesting an address\n",
            LWIP_VERSION_MAJOR, LWIP_VERSION_MINOR, LWIP_VERSION_REVISION);

    int announced = 0;

    for (;;) {
        int moved;

        lwip_sys_lock();
        moved = net_e1000_poll();
        sys_check_timeouts();

        // Report the address once, when it first arrives. Anyone who has spent
        // an evening wondering whether the guest has an address at all wants
        // this on the console rather than in a file they have to know about.
        if (!announced && !ip4_addr_isany_val(*netif_ip4_addr(nif))) {
            const ip4_addr_t *a  = netif_ip4_addr(nif);
            const ip4_addr_t *m  = netif_ip4_netmask(nif);
            const ip4_addr_t *gw = netif_ip4_gw(nif);
            cprintf("net: %d.%d.%d.%d/%d.%d.%d.%d via %d.%d.%d.%d\n",
                    ip4_addr1(a),  ip4_addr2(a),  ip4_addr3(a),  ip4_addr4(a),
                    ip4_addr1(m),  ip4_addr2(m),  ip4_addr3(m),  ip4_addr4(m),
                    ip4_addr1(gw), ip4_addr2(gw), ip4_addr3(gw), ip4_addr4(gw));
            announced = 1;
            net_bound = 1;
        }
        lwip_sys_unlock();

        // Busy while there is traffic, idle otherwise. Sleeping even 1 ms with
        // frames waiting would cap throughput at one burst per tick.
        if (moved == 0)
            wait_tick();
    }
}

// Start the stack. Called from main() once processes exist.
void
netinit(void)
{
    if (net_started)
        return;
    if (!e1000_present()) {
        // Silent: a machine without an e1000 is not misconfigured, and this
        // runs on every boot.
        return;
    }
    net_started = 1;
    if (kthread_create(net_thread, "netd") < 0)
        cprintf("net: could not start the network thread\n");
}