lguest: documentation IV: Launcher

Documentation: The Launcher

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 150 insertions, 9 deletions

diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c
index 6ae86f20ce3d..80d1b58c7698 100644
--- a/drivers/lguest/lguest_user.c
+++ b/drivers/lguest/lguest_user.c
@@ -9,33 +9,62 @@
 #include <linux/fs.h>
 #include "lg.h"
 
+/*L:030 setup_regs() doesn't really belong in this file, but it gives us an
+ * early glimpse deeper into the Host so it's worth having here.
+ *
+ * Most of the Guest's registers are left alone: we used get_zeroed_page() to
+ * allocate the structure, so they will be 0. */
 static void setup_regs(struct lguest_regs *regs, unsigned long start)
 {
-        /* Write out stack in format lguest expects, so we can switch to it. */
+        /* There are four "segment" registers which the Guest needs to boot:
+         * The "code segment" register (cs) refers to the kernel code segment
+         * __KERNEL_CS, and the "data", "extra" and "stack" segment registers
+         * refer to the kernel data segment __KERNEL_DS.
+         *
+         * The privilege level is packed into the lower bits.  The Guest runs
+         * at privilege level 1 (GUEST_PL).*/
         regs->ds = regs->es = regs->ss = __KERNEL_DS|GUEST_PL;
         regs->cs = __KERNEL_CS|GUEST_PL;
-        regs->eflags = 0x202;   /* Interrupts enabled. */
+
+        /* The "eflags" register contains miscellaneous flags.  Bit 1 (0x002)
+         * is supposed to always be "1".  Bit 9 (0x200) controls whether
+         * interrupts are enabled.  We always leave interrupts enabled while
+         * running the Guest. */
+        regs->eflags = 0x202;
+
+        /* The "Extended Instruction Pointer" register says where the Guest is
+         * running. */
         regs->eip = start;
-        /* esi points to our boot information (physical address 0) */
+
+        /* %esi points to our boot information, at physical address 0, so don't
+         * touch it. */
 }
 
-/* + addr */
+/*L:310 To send DMA into the Guest, the Launcher needs to be able to ask for a
+ * DMA buffer.  This is done by writing LHREQ_GETDMA and the key to
+ * /dev/lguest. */
 static long user_get_dma(struct lguest *lg, const u32 __user *input)
 {
         unsigned long key, udma, irq;
 
+        /* Fetch the key they wrote to us. */
         if (get_user(key, input) != 0)
                 return -EFAULT;
+        /* Look for a free Guest DMA buffer bound to that key. */
         udma = get_dma_buffer(lg, key, &irq);
         if (!udma)
                 return -ENOENT;
 
-        /* We put irq number in udma->used_len. */
+        /* We need to tell the Launcher what interrupt the Guest expects after
+         * the buffer is filled.  We stash it in udma->used_len. */
         lgwrite_u32(lg, udma + offsetof(struct lguest_dma, used_len), irq);
+
+        /* The (guest-physical) address of the DMA buffer is returned from
+         * the write(). */
         return udma;
 }
 
-/* To force the Guest to stop running and return to the Launcher, the
+/*L:315 To force the Guest to stop running and return to the Launcher, the
  * Waker sets writes LHREQ_BREAK and the value "1" to /dev/lguest.  The
  * Launcher then writes LHREQ_BREAK and "0" to release the Waker. */
 static int break_guest_out(struct lguest *lg, const u32 __user *input)
@@ -59,7 +88,8 @@ static int break_guest_out(struct lguest *lg, const u32 __user *input)
         }
 }
 
-/* + irq */
+/*L:050 Sending an interrupt is done by writing LHREQ_IRQ and an interrupt
+ * number to /dev/lguest. */
 static int user_send_irq(struct lguest *lg, const u32 __user *input)
 {
         u32 irq;
@@ -68,14 +98,19 @@ static int user_send_irq(struct lguest *lg, const u32 __user *input)
                 return -EFAULT;
         if (irq >= LGUEST_IRQS)
                 return -EINVAL;
+        /* Next time the Guest runs, the core code will see if it can deliver
+         * this interrupt. */
         set_bit(irq, lg->irqs_pending);
         return 0;
 }
 
+/*L:040 Once our Guest is initialized, the Launcher makes it run by reading
+ * from /dev/lguest. */
 static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
 {
         struct lguest *lg = file->private_data;
 
+        /* You must write LHREQ_INITIALIZE first! */
         if (!lg)
                 return -EINVAL;
 
@@ -83,27 +118,52 @@ static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
         if (current != lg->tsk)
                 return -EPERM;
 
+        /* If the guest is already dead, we indicate why */
         if (lg->dead) {
                 size_t len;
 
+                /* lg->dead either contains an error code, or a string. */
                 if (IS_ERR(lg->dead))
                         return PTR_ERR(lg->dead);
 
+                /* We can only return as much as the buffer they read with. */
                 len = min(size, strlen(lg->dead)+1);
                 if (copy_to_user(user, lg->dead, len) != 0)
                         return -EFAULT;
                 return len;
         }
 
+        /* If we returned from read() last time because the Guest sent DMA,
+         * clear the flag. */
         if (lg->dma_is_pending)
                 lg->dma_is_pending = 0;
 
+        /* Run the Guest until something interesting happens. */
         return run_guest(lg, (unsigned long __user *)user);
 }
 
-/* Take: pfnlimit, pgdir, start, pageoffset. */
+/*L:020 The initialization write supplies 4 32-bit values (in addition to the
+ * 32-bit LHREQ_INITIALIZE value).  These are:
+ *
+ * pfnlimit: The highest (Guest-physical) page number the Guest should be
+ * allowed to access.  The Launcher has to live in Guest memory, so it sets
+ * this to ensure the Guest can't reach it.
+ *
+ * pgdir: The (Guest-physical) address of the top of the initial Guest
+ * pagetables (which are set up by the Launcher).
+ *
+ * start: The first instruction to execute ("eip" in x86-speak).
+ *
+ * page_offset: The PAGE_OFFSET constant in the Guest kernel.  We should
+ * probably wean the code off this, but it's a very useful constant!  Any
+ * address above this is within the Guest kernel, and any kernel address can
+ * quickly converted from physical to virtual by adding PAGE_OFFSET.  It's
+ * 0xC0000000 (3G) by default, but it's configurable at kernel build time.
+ */
 static int initialize(struct file *file, const u32 __user *input)
 {
+        /* "struct lguest" contains everything we (the Host) know about a
+         * Guest. */
         struct lguest *lg;
         int err, i;
         u32 args[4];
@@ -111,7 +171,7 @@ static int initialize(struct file *file, const u32 __user *input)
         /* We grab the Big Lguest lock, which protects the global array
          * "lguests" and multiple simultaneous initializations. */
         mutex_lock(&lguest_lock);
-
+        /* You can't initialize twice!  Close the device and start again... */
         if (file->private_data) {
                 err = -EBUSY;
                 goto unlock;
@@ -122,37 +182,70 @@ static int initialize(struct file *file, const u32 __user *input)
                 goto unlock;
         }
 
+        /* Find an unused guest. */
         i = find_free_guest();
         if (i < 0) {
                 err = -ENOSPC;
                 goto unlock;
         }
+        /* OK, we have an index into the "lguest" array: "lg" is a convenient
+         * pointer. */
         lg = &lguests[i];
+
+        /* Populate the easy fields of our "struct lguest" */
         lg->guestid = i;
         lg->pfn_limit = args[0];
         lg->page_offset = args[3];
+
+        /* We need a complete page for the Guest registers: they are accessible
+         * to the Guest and we can only grant it access to whole pages. */
         lg->regs_page = get_zeroed_page(GFP_KERNEL);
         if (!lg->regs_page) {
                 err = -ENOMEM;
                 goto release_guest;
         }
+        /* We actually put the registers at the bottom of the page. */
         lg->regs = (void *)lg->regs_page + PAGE_SIZE - sizeof(*lg->regs);
 
+        /* Initialize the Guest's shadow page tables, using the toplevel
+         * address the Launcher gave us.  This allocates memory, so can
+         * fail. */
         err = init_guest_pagetable(lg, args[1]);
         if (err)
                 goto free_regs;
 
+        /* Now we initialize the Guest's registers, handing it the start
+         * address. */
         setup_regs(lg->regs, args[2]);
+
+        /* There are a couple of GDT entries the Guest expects when first
+         * booting. */
         setup_guest_gdt(lg);
+
+        /* The timer for lguest's clock needs initialization. */
         init_clockdev(lg);
+
+        /* We keep a pointer to the Launcher task (ie. current task) for when
+         * other Guests want to wake this one (inter-Guest I/O). */
         lg->tsk = current;
+        /* We need to keep a pointer to the Launcher's memory map, because if
+         * the Launcher dies we need to clean it up.  If we don't keep a
+         * reference, it is destroyed before close() is called. */
         lg->mm = get_task_mm(lg->tsk);
+
+        /* Initialize the queue for the waker to wait on */
         init_waitqueue_head(&lg->break_wq);
+
+        /* We remember which CPU's pages this Guest used last, for optimization
+         * when the same Guest runs on the same CPU twice. */
         lg->last_pages = NULL;
+
+        /* We keep our "struct lguest" in the file's private_data. */
         file->private_data = lg;
 
         mutex_unlock(&lguest_lock);
 
+        /* And because this is a write() call, we return the length used. */
         return sizeof(args);
 
 free_regs:
@@ -164,9 +257,15 @@ unlock:
         return err;
 }
 
+/*L:010 The first operation the Launcher does must be a write.  All writes
+ * start with a 32 bit number: for the first write this must be
+ * LHREQ_INITIALIZE to set up the Guest.  After that the Launcher can use
+ * writes of other values to get DMA buffers and send interrupts. */
 static ssize_t write(struct file *file, const char __user *input,
                      size_t size, loff_t *off)
 {
+        /* Once the guest is initialized, we hold the "struct lguest" in the
+         * file private data. */
         struct lguest *lg = file->private_data;
         u32 req;
 
@@ -174,8 +273,11 @@ static ssize_t write(struct file *file, const char __user *input,
                 return -EFAULT;
         input += sizeof(req);
 
+        /* If you haven't initialized, you must do that first. */
         if (req != LHREQ_INITIALIZE && !lg)
                 return -EINVAL;
+
+        /* Once the Guest is dead, all you can do is read() why it died. */
         if (lg && lg->dead)
                 return -ENOENT;
 
@@ -197,33 +299,72 @@ static ssize_t write(struct file *file, const char __user *input,
         }
 }
 
+/*L:060 The final piece of interface code is the close() routine.  It reverses
+ * everything done in initialize().  This is usually called because the
+ * Launcher exited.
+ *
+ * Note that the close routine returns 0 or a negative error number: it can't
+ * really fail, but it can whine.  I blame Sun for this wart, and K&R C for
+ * letting them do it. :*/
 static int close(struct inode *inode, struct file *file)
 {
         struct lguest *lg = file->private_data;
 
+        /* If we never successfully initialized, there's nothing to clean up */
         if (!lg)
                 return 0;
 
+        /* We need the big lock, to protect from inter-guest I/O and other
+         * Launchers initializing guests. */
         mutex_lock(&lguest_lock);
         /* Cancels the hrtimer set via LHCALL_SET_CLOCKEVENT. */
         hrtimer_cancel(&lg->hrt);
+        /* Free any DMA buffers the Guest had bound. */
         release_all_dma(lg);
+        /* Free up the shadow page tables for the Guest. */
         free_guest_pagetable(lg);
+        /* Now all the memory cleanups are done, it's safe to release the
+         * Launcher's memory management structure. */
         mmput(lg->mm);
+        /* If lg->dead doesn't contain an error code it will be NULL or a
+         * kmalloc()ed string, either of which is ok to hand to kfree(). */
         if (!IS_ERR(lg->dead))
                 kfree(lg->dead);
+        /* We can free up the register page we allocated. */
         free_page(lg->regs_page);
+        /* We clear the entire structure, which also marks it as free for the
+         * next user. */
         memset(lg, 0, sizeof(*lg));
+        /* Release lock and exit. */
         mutex_unlock(&lguest_lock);
+
         return 0;
 }
 
+/*L:000
+ * Welcome to our journey through the Launcher!
+ *
+ * The Launcher is the Host userspace program which sets up, runs and services
+ * the Guest.  In fact, many comments in the Drivers which refer to "the Host"
+ * doing things are inaccurate: the Launcher does all the device handling for
+ * the Guest.  The Guest can't tell what's done by the the Launcher and what by
+ * the Host.
+ *
+ * Just to confuse you: to the Host kernel, the Launcher *is* the Guest and we
+ * shall see more of that later.
+ *
+ * We begin our understanding with the Host kernel interface which the Launcher
+ * uses: reading and writing a character device called /dev/lguest.  All the
+ * work happens in the read(), write() and close() routines: */
 static struct file_operations lguest_fops = {
         .owner   = THIS_MODULE,
         .release = close,
         .write   = write,
         .read    = read,
 };
+
+/* This is a textbook example of a "misc" character device.  Populate a "struct
+ * miscdevice" and register it with misc_register(). */
 static struct miscdevice lguest_dev = {
         .minor  = MISC_DYNAMIC_MINOR,
         .name   = "lguest",