core.c

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/*P:400
 * This contains run_guest() which actually calls into the Host<->Guest
 * Switcher and analyzes the return, such as determining if the Guest wants the
 * Host to do something.  This file also contains useful helper routines.
:*/
#include <linux/module.h>
#include <linux/stringify.h>
#include <linux/stddef.h>
#include <linux/io.h>
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include <linux/cpu.h>
#include <linux/freezer.h>
#include <linux/highmem.h>
#include <linux/slab.h>
#include <asm/paravirt.h>
#include <asm/pgtable.h>
#include <asm/uaccess.h>
#include <asm/poll.h>
#include <asm/asm-offsets.h>
#include "lg.h"


static struct vm_struct *switcher_vma;
static struct page **switcher_page;

/* This One Big lock protects all inter-guest data structures. */
DEFINE_MUTEX(lguest_lock);

/*H:010
 * We need to set up the Switcher at a high virtual address.  Remember the
 * Switcher is a few hundred bytes of assembler code which actually changes the
 * CPU to run the Guest, and then changes back to the Host when a trap or
 * interrupt happens.
 *
 * The Switcher code must be at the same virtual address in the Guest as the
 * Host since it will be running as the switchover occurs.
 *
 * Trying to map memory at a particular address is an unusual thing to do, so
 * it's not a simple one-liner.
 */
static __init int map_switcher(void)
{
    int i, err;
    struct page **pagep;

    /*
     * Map the Switcher in to high memory.
     *
     * It turns out that if we choose the address 0xFFC00000 (4MB under the
     * top virtual address), it makes setting up the page tables really
     * easy.
     */

    /*
     * We allocate an array of struct page pointers.  map_vm_area() wants
     * this, rather than just an array of pages.
     */
    switcher_page = kmalloc(sizeof(switcher_page[0])*TOTAL_SWITCHER_PAGES,
                GFP_KERNEL);
    if (!switcher_page) {
        err = -ENOMEM;
        goto out;
    }

    /*
     * Now we actually allocate the pages.  The Guest will see these pages,
     * so we make sure they're zeroed.
     */
    for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) {
        switcher_page[i] = alloc_page(GFP_KERNEL|__GFP_ZERO);
        if (!switcher_page[i]) {
            err = -ENOMEM;
            goto free_some_pages;
        }
    }

    /*
     * First we check that the Switcher won't overlap the fixmap area at
     * the top of memory.  It's currently nowhere near, but it could have
     * very strange effects if it ever happened.
     */
    if (SWITCHER_ADDR + (TOTAL_SWITCHER_PAGES+1)*PAGE_SIZE > FIXADDR_START){
        err = -ENOMEM;
        printk("lguest: mapping switcher would thwack fixmap\n");
        goto free_pages;
    }

    /*
     * Now we reserve the "virtual memory area" we want: 0xFFC00000
     * (SWITCHER_ADDR).  We might not get it in theory, but in practice
     * it's worked so far.  The end address needs +1 because __get_vm_area
     * allocates an extra guard page, so we need space for that.
     */
    switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
                     VM_ALLOC, SWITCHER_ADDR, SWITCHER_ADDR
                     + (TOTAL_SWITCHER_PAGES+1) * PAGE_SIZE);
    if (!switcher_vma) {
        err = -ENOMEM;
        printk("lguest: could not map switcher pages high\n");
        goto free_pages;
    }

    /*
     * This code actually sets up the pages we've allocated to appear at
     * SWITCHER_ADDR.  map_vm_area() takes the vma we allocated above, the
     * kind of pages we're mapping (kernel pages), and a pointer to our
     * array of struct pages.  It increments that pointer, but we don't
     * care.
     */
    pagep = switcher_page;
    err = map_vm_area(switcher_vma, PAGE_KERNEL_EXEC, &pagep);
    if (err) {
        printk("lguest: map_vm_area failed: %i\n", err);
        goto free_vma;
    }

    /*
     * Now the Switcher is mapped at the right address, we can't fail!
     * Copy in the compiled-in Switcher code (from x86/switcher_32.S).
     */
    memcpy(switcher_vma->addr, start_switcher_text,
           end_switcher_text - start_switcher_text);

    printk(KERN_INFO "lguest: mapped switcher at %p\n",
           switcher_vma->addr);
    /* And we succeeded... */
    return 0;

free_vma:
    vunmap(switcher_vma->addr);
free_pages:
    i = TOTAL_SWITCHER_PAGES;
free_some_pages:
    for (--i; i >= 0; i--)
        __free_pages(switcher_page[i], 0);
    kfree(switcher_page);
out:
    return err;
}
/*:*/

/* Cleaning up the mapping when the module is unloaded is almost... too easy. */
static void unmap_switcher(void)
{
    unsigned int i;

    /* vunmap() undoes *both* map_vm_area() and __get_vm_area(). */
    vunmap(switcher_vma->addr);
    /* Now we just need to free the pages we copied the switcher into */
    for (i = 0; i < TOTAL_SWITCHER_PAGES; i++)
        __free_pages(switcher_page[i], 0);
    kfree(switcher_page);
}

/*H:032
 * Dealing With Guest Memory.
 *
 * Before we go too much further into the Host, we need to grok the routines
 * we use to deal with Guest memory.
 *
 * When the Guest gives us (what it thinks is) a physical address, we can use
 * the normal copy_from_user() & copy_to_user() on the corresponding place in
 * the memory region allocated by the Launcher.
 *
 * But we can't trust the Guest: it might be trying to access the Launcher
 * code.  We have to check that the range is below the pfn_limit the Launcher
 * gave us.  We have to make sure that addr + len doesn't give us a false
 * positive by overflowing, too.
 */
bool lguest_address_ok(const struct lguest *lg,
               unsigned long addr, unsigned long len)
{
    return (addr+len) / PAGE_SIZE < lg->pfn_limit && (addr+len >= addr);
}

/*
 * This routine copies memory from the Guest.  Here we can see how useful the
 * kill_lguest() routine we met in the Launcher can be: we return a random
 * value (all zeroes) instead of needing to return an error.
 */
void __lgread(struct lg_cpu *cpu, void *b, unsigned long addr, unsigned bytes)
{
    if (!lguest_address_ok(cpu->lg, addr, bytes)
        || copy_from_user(b, cpu->lg->mem_base + addr, bytes) != 0) {
        /* copy_from_user should do this, but as we rely on it... */
        memset(b, 0, bytes);
        kill_guest(cpu, "bad read address %#lx len %u", addr, bytes);
    }
}

/* This is the write (copy into Guest) version. */
void __lgwrite(struct lg_cpu *cpu, unsigned long addr, const void *b,
           unsigned bytes)
{
    if (!lguest_address_ok(cpu->lg, addr, bytes)
        || copy_to_user(cpu->lg->mem_base + addr, b, bytes) != 0)
        kill_guest(cpu, "bad write address %#lx len %u", addr, bytes);
}
/*:*/

/*H:030
 * Let's jump straight to the the main loop which runs the Guest.
 * Remember, this is called by the Launcher reading /dev/lguest, and we keep
 * going around and around until something interesting happens.
 */
int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
{
    /* We stop running once the Guest is dead. */
    while (!cpu->lg->dead) {
        unsigned int irq;
        bool more;

        /* First we run any hypercalls the Guest wants done. */
        if (cpu->hcall)
            do_hypercalls(cpu);

        /*
         * It's possible the Guest did a NOTIFY hypercall to the
         * Launcher.
         */
        if (cpu->pending_notify) {
            /*
             * Does it just needs to write to a registered
             * eventfd (ie. the appropriate virtqueue thread)?
             */
            if (!send_notify_to_eventfd(cpu)) {
                /* OK, we tell the main Laucher. */
                if (put_user(cpu->pending_notify, user))
                    return -EFAULT;
                return sizeof(cpu->pending_notify);
            }
        }

        /*
         * All long-lived kernel loops need to check with this horrible
         * thing called the freezer.  If the Host is trying to suspend,
         * it stops us.
         */
        try_to_freeze();

        /* Check for signals */
        if (signal_pending(current))
            return -ERESTARTSYS;

        /*
         * Check if there are any interrupts which can be delivered now:
         * if so, this sets up the hander to be executed when we next
         * run the Guest.
         */
        irq = interrupt_pending(cpu, &more);
        if (irq < LGUEST_IRQS)
            try_deliver_interrupt(cpu, irq, more);

        /*
         * Just make absolutely sure the Guest is still alive.  One of
         * those hypercalls could have been fatal, for example.
         */
        if (cpu->lg->dead)
            break;

        /*
         * If the Guest asked to be stopped, we sleep.  The Guest's
         * clock timer will wake us.
         */
        if (cpu->halted) {
            set_current_state(TASK_INTERRUPTIBLE);
            /*
             * Just before we sleep, make sure no interrupt snuck in
             * which we should be doing.
             */
            if (interrupt_pending(cpu, &more) < LGUEST_IRQS)
                set_current_state(TASK_RUNNING);
            else
                schedule();
            continue;
        }

        /*
         * OK, now we're ready to jump into the Guest.  First we put up
         * the "Do Not Disturb" sign:
         */
        local_irq_disable();

        /* Actually run the Guest until something happens. */
        lguest_arch_run_guest(cpu);

        /* Now we're ready to be interrupted or moved to other CPUs */
        local_irq_enable();

        /* Now we deal with whatever happened to the Guest. */
        lguest_arch_handle_trap(cpu);
    }

    /* Special case: Guest is 'dead' but wants a reboot. */
    if (cpu->lg->dead == ERR_PTR(-ERESTART))
        return -ERESTART;

    /* The Guest is dead => "No such file or directory" */
    return -ENOENT;
}

/*H:000
 * Welcome to the Host!
 *
 * By this point your brain has been tickled by the Guest code and numbed by
 * the Launcher code; prepare for it to be stretched by the Host code.  This is
 * the heart.  Let's begin at the initialization routine for the Host's lg
 * module.
 */
static int __init init(void)
{
    int err;

    /* Lguest can't run under Xen, VMI or itself.  It does Tricky Stuff. */
    if (get_kernel_rpl() != 0) {
        printk("lguest is afraid of being a guest\n");
        return -EPERM;
    }

    /* First we put the Switcher up in very high virtual memory. */
    err = map_switcher();
    if (err)
        goto out;

    /* Now we set up the pagetable implementation for the Guests. */
    err = init_pagetables(switcher_page, SHARED_SWITCHER_PAGES);
    if (err)
        goto unmap;

    /* We might need to reserve an interrupt vector. */
    err = init_interrupts();
    if (err)
        goto free_pgtables;

    /* /dev/lguest needs to be registered. */
    err = lguest_device_init();
    if (err)
        goto free_interrupts;

    /* Finally we do some architecture-specific setup. */
    lguest_arch_host_init();

    /* All good! */
    return 0;

free_interrupts:
    free_interrupts();
free_pgtables:
    free_pagetables();
unmap:
    unmap_switcher();
out:
    return err;
}

/* Cleaning up is just the same code, backwards.  With a little French. */
static void __exit fini(void)
{
    lguest_device_remove();
    free_interrupts();
    free_pagetables();
    unmap_switcher();

    lguest_arch_host_fini();
}
/*:*/

/*
 * The Host side of lguest can be a module.  This is a nice way for people to
 * play with it.
 */
module_init(init);
module_exit(fini);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Rusty Russell <[email protected]>");