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core.c

/*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, and a
 * couple of non-obvious setup and teardown pieces which were implemented after
 * days of debugging pain. :*/
#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 <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"s.  map_vm_area() wants the
       * pages in this form, rather than just an array of pointers. */
      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++) {
            unsigned long addr = get_zeroed_page(GFP_KERNEL);
            if (!addr) {
                  err = -ENOMEM;
                  goto free_some_pages;
            }
            switcher_page[i] = virt_to_page(addr);
      }

      /* 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. */
      switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
                               VM_ALLOC, SWITCHER_ADDR, VMALLOC_END);
      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, &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 <arch>_switcher.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);
}

/*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. */
int 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 lguest *lg, void *b, unsigned long addr, unsigned bytes)
{
      if (!lguest_address_ok(lg, addr, bytes)
          || copy_from_user(b, lg->mem_base + addr, bytes) != 0) {
            /* copy_from_user should do this, but as we rely on it... */
            memset(b, 0, bytes);
            kill_guest(lg, "bad read address %#lx len %u", addr, bytes);
      }
}

/* This is the write (copy into guest) version. */
void __lgwrite(struct lguest *lg, unsigned long addr, const void *b,
             unsigned bytes)
{
      if (!lguest_address_ok(lg, addr, bytes)
          || copy_to_user(lg->mem_base + addr, b, bytes) != 0)
            kill_guest(lg, "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 lguest *lg, unsigned long __user *user)
{
      /* We stop running once the Guest is dead. */
      while (!lg->dead) {
            /* First we run any hypercalls the Guest wants done. */
            if (lg->hcall)
                  do_hypercalls(lg);

            /* It's possible the Guest did a NOTIFY hypercall to the
             * Launcher, in which case we return from the read() now. */
            if (lg->pending_notify) {
                  if (put_user(lg->pending_notify, user))
                        return -EFAULT;
                  return sizeof(lg->pending_notify);
            }

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

            /* If Waker set break_out, return to Launcher. */
            if (lg->break_out)
                  return -EAGAIN;

            /* 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. */
            maybe_do_interrupt(lg);

            /* 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();

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

            /* If the Guest asked to be stopped, we sleep.  The Guest's
             * clock timer or LHCALL_BREAK from the Waker will wake us. */
            if (lg->halted) {
                  set_current_state(TASK_INTERRUPTIBLE);
                  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(lg);

            /* 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(lg);
      }

      /* 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 (paravirt_enabled()) {
            printk("lguest is afraid of %s\n", pv_info.name);
            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 <rusty@rustcorp.com.au>");

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