lguest: comment documentation update.

Took some cycles to re-read the Lguest Journey end-to-end, fix some
rot and tighten some phrases.

Only comments change.  No new jokes, but a couple of recycled old jokes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

8 files changed, 91 insertions, 59 deletions

diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index c632c08..5eea435 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -1,8 +1,6 @@
 /*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. :*/
+ * Host to do something.  This file also contains useful helper routines. :*/
 #include <linux/module.h>
 #include <linux/stringify.h>
 #include <linux/stddef.h>
@@ -49,8 +47,8 @@ static __init int map_switcher(void)
 	 * 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. */
+	/* 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) {
@@ -172,7 +170,7 @@ void __lgread(struct lg_cpu *cpu, void *b, unsigned long addr, unsigned bytes)
 	}
 }
 
-/* This is the write (copy into guest) version. */
+/* This is the write (copy into Guest) version. */
 void __lgwrite(struct lg_cpu *cpu, unsigned long addr, const void *b,
 	       unsigned bytes)
 {
@@ -209,9 +207,9 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 		if (cpu->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. */
+		/* 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(cpu);
 
 		/* All long-lived kernel loops need to check with this horrible
@@ -246,8 +244,10 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 		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;
 }
diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index 0f2cb4f..54d66f0 100644
--- a/drivers/lguest/hypercalls.c
+++ b/drivers/lguest/hypercalls.c
@@ -29,7 +29,7 @@
 #include "lg.h"
 
 /*H:120 This is the core hypercall routine: where the Guest gets what it wants.
- * Or gets killed.  Or, in the case of LHCALL_CRASH, both. */
+ * Or gets killed.  Or, in the case of LHCALL_SHUTDOWN, both. */
 static void do_hcall(struct lg_cpu *cpu, struct hcall_args *args)
 {
 	switch (args->arg0) {
@@ -190,6 +190,13 @@ static void initialize(struct lg_cpu *cpu)
 	 * pagetable. */
 	guest_pagetable_clear_all(cpu);
 }
+/*:*/
+
+/*M:013 If a Guest reads from a page (so creates a mapping) that it has never
+ * written to, and then the Launcher writes to it (ie. the output of a virtual
+ * device), the Guest will still see the old page.  In practice, this never
+ * happens: why would the Guest read a page which it has never written to?  But
+ * a similar scenario might one day bite us, so it's worth mentioning. :*/
 
 /*H:100
  * Hypercalls
@@ -227,7 +234,7 @@ void do_hypercalls(struct lg_cpu *cpu)
 		 * However, if we are signalled or the Guest sends I/O to the
 		 * Launcher, the run_guest() loop will exit without running the
 		 * Guest.  When it comes back it would try to re-run the
-		 * hypercall. */
+		 * hypercall.  Finding that bug sucked. */
 		cpu->hcall = NULL;
 	}
 }
diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c
index 32e97c1..0414ddf 100644
--- a/drivers/lguest/interrupts_and_traps.c
+++ b/drivers/lguest/interrupts_and_traps.c
@@ -144,7 +144,6 @@ void maybe_do_interrupt(struct lg_cpu *cpu)
 	if (copy_from_user(&blk, cpu->lg->lguest_data->blocked_interrupts,
 			   sizeof(blk)))
 		return;
-
 	bitmap_andnot(blk, cpu->irqs_pending, blk, LGUEST_IRQS);
 
 	/* Find the first interrupt. */
@@ -237,9 +236,9 @@ void free_interrupts(void)
 		clear_bit(syscall_vector, used_vectors);
 }
 
-/*H:220 Now we've got the routines to deliver interrupts, delivering traps
- * like page fault is easy.  The only trick is that Intel decided that some
- * traps should have error codes: */
+/*H:220 Now we've got the routines to deliver interrupts, delivering traps like
+ * page fault is easy.  The only trick is that Intel decided that some traps
+ * should have error codes: */
 static int has_err(unsigned int trap)
 {
 	return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17);
diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index 1b2ec0b..2bc9bf7 100644
--- a/drivers/lguest/lguest_device.c
+++ b/drivers/lguest/lguest_device.c
@@ -1,10 +1,10 @@
 /*P:050 Lguest guests use a very simple method to describe devices.  It's a
- * series of device descriptors contained just above the top of normal
+ * series of device descriptors contained just above the top of normal Guest
  * memory.
  *
  * We use the standard "virtio" device infrastructure, which provides us with a
  * console, a network and a block driver.  Each one expects some configuration
- * information and a "virtqueue" mechanism to send and receive data. :*/
+ * information and a "virtqueue" or two to send and receive data. :*/
 #include <linux/init.h>
 #include <linux/bootmem.h>
 #include <linux/lguest_launcher.h>
@@ -53,7 +53,7 @@ struct lguest_device {
  * Device configurations
  *
  * The configuration information for a device consists of one or more
- * virtqueues, a feature bitmaks, and some configuration bytes.  The
+ * virtqueues, a feature bitmap, and some configuration bytes.  The
  * configuration bytes don't really matter to us: the Launcher sets them up, and
  * the driver will look at them during setup.
  *
@@ -179,7 +179,7 @@ struct lguest_vq_info
 };
 
 /* When the virtio_ring code wants to prod the Host, it calls us here and we
- * make a hypercall.  We hand the page number of the virtqueue so the Host
+ * make a hypercall.  We hand the physical address of the virtqueue so the Host
  * knows which virtqueue we're talking about. */
 static void lg_notify(struct virtqueue *vq)
 {
@@ -199,7 +199,8 @@ static void lg_notify(struct virtqueue *vq)
  * allocate its own pages and tell the Host where they are, but for lguest it's
  * simpler for the Host to simply tell us where the pages are.
  *
- * So we provide devices with a "find virtqueue and set it up" function. */
+ * So we provide drivers with a "find the Nth virtqueue and set it up"
+ * function. */
 static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 				    unsigned index,
 				    void (*callback)(struct virtqueue *vq))
diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c
index 2221485..564e425 100644
--- a/drivers/lguest/lguest_user.c
+++ b/drivers/lguest/lguest_user.c
@@ -73,7 +73,7 @@ static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
 	if (current != cpu->tsk)
 		return -EPERM;
 
-	/* If the guest is already dead, we indicate why */
+	/* If the Guest is already dead, we indicate why */
 	if (lg->dead) {
 		size_t len;
 
@@ -88,7 +88,7 @@ static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
 		return len;
 	}
 
-	/* If we returned from read() last time because the Guest notified,
+	/* If we returned from read() last time because the Guest sent I/O,
 	 * clear the flag. */
 	if (cpu->pending_notify)
 		cpu->pending_notify = 0;
@@ -97,14 +97,20 @@ static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
 	return run_guest(cpu, (unsigned long __user *)user);
 }
 
+/*L:025 This actually initializes a CPU.  For the moment, a Guest is only
+ * uniprocessor, so "id" is always 0. */
 static int lg_cpu_start(struct lg_cpu *cpu, unsigned id, unsigned long start_ip)
 {
+	/* We have a limited number the number of CPUs in the lguest struct. */
 	if (id >= NR_CPUS)
 		return -EINVAL;
 
+	/* Set up this CPU's id, and pointer back to the lguest struct. */
 	cpu->id = id;
 	cpu->lg = container_of((cpu - id), struct lguest, cpus[0]);
 	cpu->lg->nr_cpus++;
+
+	/* Each CPU has a timer it can set. */
 	init_clockdev(cpu);
 
 	/* We need a complete page for the Guest registers: they are accessible
@@ -120,11 +126,11 @@ static int lg_cpu_start(struct lg_cpu *cpu, unsigned id, unsigned long start_ip)
 	 * address. */
 	lguest_arch_setup_regs(cpu, start_ip);
 
-	/* Initialize the queue for the waker to wait on */
+	/* Initialize the queue for the Waker to wait on */
 	init_waitqueue_head(&cpu->break_wq);
 
 	/* We keep a pointer to the Launcher task (ie. current task) for when
-	 * other Guests want to wake this one (inter-Guest I/O). */
+	 * other Guests want to wake this one (eg. console input). */
 	cpu->tsk = current;
 
 	/* We need to keep a pointer to the Launcher's memory map, because if
@@ -136,6 +142,7 @@ static int lg_cpu_start(struct lg_cpu *cpu, unsigned id, unsigned long start_ip)
 	 * when the same Guest runs on the same CPU twice. */
 	cpu->last_pages = NULL;
 
+	/* No error == success. */
 	return 0;
 }
 
@@ -185,14 +192,13 @@ static int initialize(struct file *file, const unsigned long __user *input)
 	lg->mem_base = (void __user *)(long)args[0];
 	lg->pfn_limit = args[1];
 
-	/* This is the first cpu */
+	/* This is the first cpu (cpu 0) and it will start booting at args[3] */
 	err = lg_cpu_start(&lg->cpus[0], 0, args[3]);
 	if (err)
 		goto release_guest;
 
 	/* Initialize the Guest's shadow page tables, using the toplevel
-	 * address the Launcher gave us.  This allocates memory, so can
-	 * fail. */
+	 * address the Launcher gave us.  This allocates memory, so can fail. */
 	err = init_guest_pagetable(lg, args[2]);
 	if (err)
 		goto free_regs;
@@ -218,11 +224,16 @@ unlock:
 /*L:010 The first operation the Launcher does must be a write.  All writes
  * start with an unsigned long 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 send interrupts. */
+ * writes of other values to send interrupts.
+ *
+ * Note that we overload the "offset" in the /dev/lguest file to indicate what
+ * CPU number we're dealing with.  Currently this is always 0, since we only
+ * support uniprocessor Guests, but you can see the beginnings of SMP support
+ * here. */
 static ssize_t write(struct file *file, const char __user *in,
 		     size_t size, loff_t *off)
 {
-	/* Once the guest is initialized, we hold the "struct lguest" in the
+	/* Once the Guest is initialized, we hold the "struct lguest" in the
 	 * file private data. */
 	struct lguest *lg = file->private_data;
 	const unsigned long __user *input = (const unsigned long __user *)in;
@@ -230,6 +241,7 @@ static ssize_t write(struct file *file, const char __user *in,
 	struct lg_cpu *uninitialized_var(cpu);
 	unsigned int cpu_id = *off;
 
+	/* The first value tells us what this request is. */
 	if (get_user(req, input) != 0)
 		return -EFAULT;
 	input++;
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index a7f64a9..d93500f 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -2,8 +2,8 @@
  * previous encounters.  It's functional, and as neat as it can be in the
  * circumstances, but be wary, for these things are subtle and break easily.
  * The Guest provides a virtual to physical mapping, but we can neither trust
- * it nor use it: we verify and convert it here to point the hardware to the
- * actual Guest pages when running the Guest. :*/
+ * it nor use it: we verify and convert it here then point the CPU to the
+ * converted Guest pages when running the Guest. :*/
 
 /* Copyright (C) Rusty Russell IBM Corporation 2006.
  * GPL v2 and any later version */
@@ -106,6 +106,11 @@ static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr)
 	BUG_ON(!(pgd_flags(gpgd) & _PAGE_PRESENT));
 	return gpage + ((vaddr>>PAGE_SHIFT) % PTRS_PER_PTE) * sizeof(pte_t);
 }
+/*:*/
+
+/*M:014 get_pfn is slow; it takes the mmap sem and calls get_user_pages.  We
+ * could probably try to grab batches of pages here as an optimization
+ * (ie. pre-faulting). :*/
 
 /*H:350 This routine takes a page number given by the Guest and converts it to
  * an actual, physical page number.  It can fail for several reasons: the
@@ -113,8 +118,8 @@ static unsigned long gpte_addr(pgd_t gpgd, unsigned long vaddr)
  * and the page is read-only, or the write flag was set and the page was
  * shared so had to be copied, but we ran out of memory.
  *
- * This holds a reference to the page, so release_pte() is careful to
- * put that back. */
+ * This holds a reference to the page, so release_pte() is careful to put that
+ * back. */
 static unsigned long get_pfn(unsigned long virtpfn, int write)
 {
 	struct page *page;
@@ -532,13 +537,13 @@ static void do_set_pte(struct lg_cpu *cpu, int idx,
  * all processes.  So when the page table above that address changes, we update
  * all the page tables, not just the current one.  This is rare.
  *
- * The benefit is that when we have to track a new page table, we can copy keep
- * all the kernel mappings.  This speeds up context switch immensely. */
+ * The benefit is that when we have to track a new page table, we can keep all
+ * the kernel mappings.  This speeds up context switch immensely. */
 void guest_set_pte(struct lg_cpu *cpu,
 		   unsigned long gpgdir, unsigned long vaddr, pte_t gpte)
 {
-	/* Kernel mappings must be changed on all top levels.  Slow, but
-	 * doesn't happen often. */
+	/* Kernel mappings must be changed on all top levels.  Slow, but doesn't
+	 * happen often. */
 	if (vaddr >= cpu->lg->kernel_address) {
 		unsigned int i;
 		for (i = 0; i < ARRAY_SIZE(cpu->lg->pgdirs); i++)
@@ -704,12 +709,11 @@ static __init void populate_switcher_pte_page(unsigned int cpu,
 /* We've made it through the page table code.  Perhaps our tired brains are
  * still processing the details, or perhaps we're simply glad it's over.
  *
- * If nothing else, note that all this complexity in juggling shadow page
- * tables in sync with the Guest's page tables is for one reason: for most
- * Guests this page table dance determines how bad performance will be.  This
- * is why Xen uses exotic direct Guest pagetable manipulation, and why both
- * Intel and AMD have implemented shadow page table support directly into
- * hardware.
+ * If nothing else, note that all this complexity in juggling shadow page tables
+ * in sync with the Guest's page tables is for one reason: for most Guests this
+ * page table dance determines how bad performance will be.  This is why Xen
+ * uses exotic direct Guest pagetable manipulation, and why both Intel and AMD
+ * have implemented shadow page table support directly into hardware.
  *
  * There is just one file remaining in the Host. */
 
diff --git a/drivers/lguest/x86/core.c b/drivers/lguest/x86/core.c
index 6351878..5126d5d 100644
--- a/drivers/lguest/x86/core.c
+++ b/drivers/lguest/x86/core.c
@@ -17,6 +17,13 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
+/*P:450 This file contains the x86-specific lguest code.  It used to be all
+ * mixed in with drivers/lguest/core.c but several foolhardy code slashers
+ * wrestled most of the dependencies out to here in preparation for porting
+ * lguest to other architectures (see what I mean by foolhardy?).
+ *
+ * This also contains a couple of non-obvious setup and teardown pieces which
+ * were implemented after days of debugging pain. :*/
 #include <linux/kernel.h>
 #include <linux/start_kernel.h>
 #include <linux/string.h>
@@ -157,6 +164,8 @@ static void run_guest_once(struct lg_cpu *cpu, struct lguest_pages *pages)
  * also simplify copy_in_guest_info().  Note that we'd still need to restore
  * things when we exit to Launcher userspace, but that's fairly easy.
  *
+ * We could also try using this hooks for PGE, but that might be too expensive.
+ *
  * The hooks were designed for KVM, but we can also put them to good use. :*/
 
 /*H:040 This is the i386-specific code to setup and run the Guest.  Interrupts
@@ -182,7 +191,7 @@ void lguest_arch_run_guest(struct lg_cpu *cpu)
 	 * was doing. */
 	run_guest_once(cpu, lguest_pages(raw_smp_processor_id()));
 
-	/* Note that the "regs" pointer contains two extra entries which are
+	/* Note that the "regs" structure contains two extra entries which are
 	 * not really registers: a trap number which says what interrupt or
 	 * trap made the switcher code come back, and an error code which some
 	 * traps set.  */
@@ -293,11 +302,10 @@ void lguest_arch_handle_trap(struct lg_cpu *cpu)
 		break;
 	case 14: /* We've intercepted a Page Fault. */
 		/* The Guest accessed a virtual address that wasn't mapped.
-		 * This happens a lot: we don't actually set up most of the
-		 * page tables for the Guest at all when we start: as it runs
-		 * it asks for more and more, and we set them up as
-		 * required. In this case, we don't even tell the Guest that
-		 * the fault happened.
+		 * This happens a lot: we don't actually set up most of the page
+		 * tables for the Guest at all when we start: as it runs it asks
+		 * for more and more, and we set them up as required. In this
+		 * case, we don't even tell the Guest that the fault happened.
 		 *
 		 * The errcode tells whether this was a read or a write, and
 		 * whether kernel or userspace code. */
@@ -342,7 +350,7 @@ void lguest_arch_handle_trap(struct lg_cpu *cpu)
 	if (!deliver_trap(cpu, cpu->regs->trapnum))
 		/* If the Guest doesn't have a handler (either it hasn't
 		 * registered any yet, or it's one of the faults we don't let
-		 * it handle), it dies with a cryptic error message. */
+		 * it handle), it dies with this cryptic error message. */
 		kill_guest(cpu, "unhandled trap %li at %#lx (%#lx)",
 			   cpu->regs->trapnum, cpu->regs->eip,
 			   cpu->regs->trapnum == 14 ? cpu->arch.last_pagefault
@@ -375,8 +383,8 @@ void __init lguest_arch_host_init(void)
 	 * The only exception is the interrupt handlers in switcher.S: their
 	 * addresses are placed in a table (default_idt_entries), so we need to
 	 * update the table with the new addresses.  switcher_offset() is a
-	 * convenience function which returns the distance between the builtin
-	 * switcher code and the high-mapped copy we just made. */
+	 * convenience function which returns the distance between the
+	 * compiled-in switcher code and the high-mapped copy we just made. */
 	for (i = 0; i < IDT_ENTRIES; i++)
 		default_idt_entries[i] += switcher_offset();
 
@@ -416,7 +424,7 @@ void __init lguest_arch_host_init(void)
 		state->guest_gdt_desc.address = (long)&state->guest_gdt;
 
 		/* We know where we want the stack to be when the Guest enters
-		 * the switcher: in pages->regs.  The stack grows upwards, so
+		 * the Switcher: in pages->regs.  The stack grows upwards, so
 		 * we start it at the end of that structure. */
 		state->guest_tss.sp0 = (long)(&pages->regs + 1);
 		/* And this is the GDT entry to use for the stack: we keep a
@@ -513,8 +521,8 @@ int lguest_arch_init_hypercalls(struct lg_cpu *cpu)
 {
 	u32 tsc_speed;
 
-	/* The pointer to the Guest's "struct lguest_data" is the only
-	 * argument.  We check that address now. */
+	/* The pointer to the Guest's "struct lguest_data" is the only argument.
+	 * We check that address now. */
 	if (!lguest_address_ok(cpu->lg, cpu->hcall->arg1,
 			       sizeof(*cpu->lg->lguest_data)))
 		return -EFAULT;
@@ -546,6 +554,7 @@ int lguest_arch_init_hypercalls(struct lg_cpu *cpu)
 
 	return 0;
 }
+/*:*/
 
 /*L:030 lguest_arch_setup_regs()
  *
diff --git a/drivers/lguest/x86/switcher_32.S b/drivers/lguest/x86/switcher_32.S
index 0af8baa..3fc1531 100644
--- a/drivers/lguest/x86/switcher_32.S
+++ b/drivers/lguest/x86/switcher_32.S
@@ -1,6 +1,6 @@
-/*P:900 This is the Switcher: code which sits at 0xFFC00000 to do the low-level
- * Guest<->Host switch.  It is as simple as it can be made, but it's naturally
- * very specific to x86.
+/*P:900 This is the Switcher: code which sits at 0xFFC00000 astride both the
+ * Host and Guest to do the low-level Guest<->Host switch.  It is as simple as
+ * it can be made, but it's naturally very specific to x86.
  *
  * You have now completed Preparation.  If this has whet your appetite; if you
  * are feeling invigorated and refreshed then the next, more challenging stage
@@ -189,7 +189,7 @@ ENTRY(switch_to_guest)
 	// Interrupts are turned back on: we are Guest.
 	iret
 
-// We treat two paths to switch back to the Host
+// We tread two paths to switch back to the Host
 // Yet both must save Guest state and restore Host
 // So we put the routine in a macro.
 #define SWITCH_TO_HOST							\