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>

drivers/lguest/core.c

@@ -208,24 +208,39 @@ static int emulate_insn(struct lguest *lg)
 	return 1;
 }
 
+/*L:305
+ * Dealing 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 that address: remember,
+ * Guest physical == Launcher virtual.
+ *
+ * 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);
 }
 
-/* Just like get_user, but don't let guest access lguest binary. */
+/* This is a convenient routine to get a 32-bit value from the Guest (a very
+ * common operation).  Here we can see how useful the kill_lguest() routine we
+ * met in the Launcher can be: we return a random value (0) instead of needing
+ * to return an error. */
 u32 lgread_u32(struct lguest *lg, unsigned long addr)
 {
 	u32 val = 0;
 
-	/* Don't let them access lguest binary */
+	/* Don't let them access lguest binary. */
 	if (!lguest_address_ok(lg, addr, sizeof(val))
 	    || get_user(val, (u32 __user *)addr) != 0)
 		kill_guest(lg, "bad read address %#lx", addr);
 	return val;
 }
 
+/* Same thing for writing a value. */
 void lgwrite_u32(struct lguest *lg, unsigned long addr, u32 val)
 {
 	if (!lguest_address_ok(lg, addr, sizeof(val))
@@ -233,6 +248,9 @@ void lgwrite_u32(struct lguest *lg, unsigned long addr, u32 val)
 		kill_guest(lg, "bad write address %#lx", addr);
 }
 
+/* This routine is more generic, and copies a range of Guest bytes into a
+ * buffer.  If the copy_from_user() fails, we fill the buffer with zeroes, so
+ * the caller doesn't end up using uninitialized kernel memory. */
 void lgread(struct lguest *lg, void *b, unsigned long addr, unsigned bytes)
 {
 	if (!lguest_address_ok(lg, addr, bytes)
@@ -243,6 +261,7 @@ void lgread(struct lguest *lg, void *b, unsigned long addr, unsigned bytes)
 	}
 }
 
+/* Similarly, our generic routine to copy into a range of Guest bytes. */
 void lgwrite(struct lguest *lg, unsigned long addr, const void *b,
 	     unsigned bytes)
 {
@@ -250,6 +269,7 @@ void lgwrite(struct lguest *lg, unsigned long addr, const void *b,
 	    || copy_to_user((void __user *)addr, b, bytes) != 0)
 		kill_guest(lg, "bad write address %#lx len %u", addr, bytes);
 }
+/* (end of memory access helper routines) :*/
 
 static void set_ts(void)
 {

drivers/lguest/lg.h

@@ -244,6 +244,30 @@ unsigned long get_dma_buffer(struct lguest *lg, unsigned long key,
 /* hypercalls.c: */
 void do_hypercalls(struct lguest *lg);
 
+/*L:035
+ * Let's step aside for the moment, to study one important routine that's used
+ * widely in the Host code.
+ *
+ * There are many cases where the Guest does something invalid, like pass crap
+ * to a hypercall.  Since only the Guest kernel can make hypercalls, it's quite
+ * acceptable to simply terminate the Guest and give the Launcher a nicely
+ * formatted reason.  It's also simpler for the Guest itself, which doesn't
+ * need to check most hypercalls for "success"; if you're still running, it
+ * succeeded.
+ *
+ * Once this is called, the Guest will never run again, so most Host code can
+ * call this then continue as if nothing had happened.  This means many
+ * functions don't have to explicitly return an error code, which keeps the
+ * code simple.
+ *
+ * It also means that this can be called more than once: only the first one is
+ * remembered.  The only trick is that we still need to kill the Guest even if
+ * we can't allocate memory to store the reason.  Linux has a neat way of
+ * packing error codes into invalid pointers, so we use that here.
+ *
+ * Like any macro which uses an "if", it is safely wrapped in a run-once "do {
+ * } while(0)".
+ */
 #define kill_guest(lg, fmt...)					\
 do {								\
 	if (!(lg)->dead) {					\
@@ -252,6 +276,7 @@ do {								\
 			(lg)->dead = ERR_PTR(-ENOMEM);		\
 	}							\
 } while(0)
+/* (End of aside) :*/
 
 static inline unsigned long guest_pa(struct lguest *lg, unsigned long vaddr)
 {

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",