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>
diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index d45c7f6..4c1fc65 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -1,7 +1,7 @@
/*P:100 This is the Launcher code, a simple program which lays out the
- * "physical" memory for the new Guest by mapping the kernel image and the
- * virtual devices, then reads repeatedly from /dev/lguest to run the Guest.
-:*/
+ * "physical" memory for the new Guest by mapping the kernel image and
+ * the virtual devices, then opens /dev/lguest to tell the kernel
+ * about the Guest and control it. :*/
#define _LARGEFILE64_SOURCE
#define _GNU_SOURCE
#include <stdio.h>
@@ -43,7 +43,7 @@
#include "linux/virtio_console.h"
#include "linux/virtio_ring.h"
#include "asm-x86/bootparam.h"
-/*L:110 We can ignore the 38 include files we need for this program, but I do
+/*L:110 We can ignore the 39 include files we need for this program, but I do
* want to draw attention to the use of kernel-style types.
*
* As Linus said, "C is a Spartan language, and so should your naming be." I
@@ -320,7 +320,7 @@
err(1, "Reading program headers");
/* Try all the headers: there are usually only three. A read-only one,
- * a read-write one, and a "note" section which isn't loadable. */
+ * a read-write one, and a "note" section which we don't load. */
for (i = 0; i < ehdr->e_phnum; i++) {
/* If this isn't a loadable segment, we ignore it */
if (phdr[i].p_type != PT_LOAD)
@@ -387,7 +387,7 @@
if (memcmp(hdr.e_ident, ELFMAG, SELFMAG) == 0)
return map_elf(fd, &hdr);
- /* Otherwise we assume it's a bzImage, and try to unpack it */
+ /* Otherwise we assume it's a bzImage, and try to load it. */
return load_bzimage(fd);
}
@@ -433,12 +433,12 @@
return len;
}
-/* Once we know how much memory we have, we can construct simple linear page
+/* Once we know how much memory we have we can construct simple linear page
* tables which set virtual == physical which will get the Guest far enough
* into the boot to create its own.
*
* We lay them out of the way, just below the initrd (which is why we need to
- * know its size). */
+ * know its size here). */
static unsigned long setup_pagetables(unsigned long mem,
unsigned long initrd_size)
{
@@ -850,7 +850,8 @@
*
* Handling output for network is also simple: we get all the output buffers
* and write them (ignoring the first element) to this device's file descriptor
- * (stdout). */
+ * (/dev/net/tun).
+ */
static void handle_net_output(int fd, struct virtqueue *vq)
{
unsigned int head, out, in;
@@ -924,7 +925,7 @@
write(waker_fd, &vq->dev->fd, sizeof(vq->dev->fd));
}
-/* Resetting a device is fairly easy. */
+/* When the Guest asks us to reset a device, it's is fairly easy. */
static void reset_device(struct device *dev)
{
struct virtqueue *vq;
@@ -1003,8 +1004,8 @@
if (select(devices.max_infd+1, &fds, NULL, NULL, &poll) == 0)
break;
- /* Otherwise, call the device(s) which have readable
- * file descriptors and a method of handling them. */
+ /* Otherwise, call the device(s) which have readable file
+ * descriptors and a method of handling them. */
for (i = devices.dev; i; i = i->next) {
if (i->handle_input && FD_ISSET(i->fd, &fds)) {
int dev_fd;
@@ -1015,8 +1016,7 @@
* should no longer service it. Networking and
* console do this when there's no input
* buffers to deliver into. Console also uses
- * it when it discovers that stdin is
- * closed. */
+ * it when it discovers that stdin is closed. */
FD_CLR(i->fd, &devices.infds);
/* Tell waker to ignore it too, by sending a
* negative fd number (-1, since 0 is a valid
@@ -1033,7 +1033,8 @@
*
* All devices need a descriptor so the Guest knows it exists, and a "struct
* device" so the Launcher can keep track of it. We have common helper
- * routines to allocate and manage them. */
+ * routines to allocate and manage them.
+ */
/* The layout of the device page is a "struct lguest_device_desc" followed by a
* number of virtqueue descriptors, then two sets of feature bits, then an
@@ -1078,7 +1079,7 @@
struct virtqueue **i, *vq = malloc(sizeof(*vq));
void *p;
- /* First we need some pages for this virtqueue. */
+ /* First we need some memory for this virtqueue. */
pages = (vring_size(num_descs, getpagesize()) + getpagesize() - 1)
/ getpagesize();
p = get_pages(pages);
@@ -1122,7 +1123,7 @@
}
/* The first half of the feature bitmask is for us to advertise features. The
- * second half if for the Guest to accept features. */
+ * second half is for the Guest to accept features. */
static void add_feature(struct device *dev, unsigned bit)
{
u8 *features = get_feature_bits(dev);
@@ -1151,7 +1152,9 @@
}
/* This routine does all the creation and setup of a new device, including
- * calling new_dev_desc() to allocate the descriptor and device memory. */
+ * calling new_dev_desc() to allocate the descriptor and device memory.
+ *
+ * See what I mean about userspace being boring? */
static struct device *new_device(const char *name, u16 type, int fd,
bool (*handle_input)(int, struct device *))
{
@@ -1492,7 +1495,10 @@
while (read(vblk->workpipe[0], &c, 1) == 1) {
/* We acknowledge each request immediately to reduce latency,
* rather than waiting until we've done them all. I haven't
- * measured to see if it makes any difference. */
+ * measured to see if it makes any difference.
+ *
+ * That would be an interesting test, wouldn't it? You could
+ * also try having more than one I/O thread. */
while (service_io(dev))
write(vblk->done_fd, &c, 1);
}
@@ -1500,7 +1506,7 @@
}
/* Now we've seen the I/O thread, we return to the Launcher to see what happens
- * when the thread tells us it's completed some I/O. */
+ * when that thread tells us it's completed some I/O. */
static bool handle_io_finish(int fd, struct device *dev)
{
char c;
@@ -1572,11 +1578,12 @@
* more work. */
pipe(vblk->workpipe);
- /* Create stack for thread and run it */
+ /* Create stack for thread and run it. Since stack grows upwards, we
+ * point the stack pointer to the end of this region. */
stack = malloc(32768);
/* SIGCHLD - We dont "wait" for our cloned thread, so prevent it from
* becoming a zombie. */
- if (clone(io_thread, stack + 32768, CLONE_VM | SIGCHLD, dev) == -1)
+ if (clone(io_thread, stack + 32768, CLONE_VM | SIGCHLD, dev) == -1)
err(1, "Creating clone");
/* We don't need to keep the I/O thread's end of the pipes open. */
@@ -1586,14 +1593,14 @@
verbose("device %u: virtblock %llu sectors\n",
devices.device_num, le64_to_cpu(conf.capacity));
}
-/* That's the end of device setup. :*/
+/* That's the end of device setup. */
-/* Reboot */
+/*L:230 Reboot is pretty easy: clean up and exec() the Launcher afresh. */
static void __attribute__((noreturn)) restart_guest(void)
{
unsigned int i;
- /* Closing pipes causes the waker thread and io_threads to die, and
+ /* Closing pipes causes the Waker thread and io_threads to die, and
* closing /dev/lguest cleans up the Guest. Since we don't track all
* open fds, we simply close everything beyond stderr. */
for (i = 3; i < FD_SETSIZE; i++)
@@ -1602,7 +1609,7 @@
err(1, "Could not exec %s", main_args[0]);
}
-/*L:220 Finally we reach the core of the Launcher, which runs the Guest, serves
+/*L:220 Finally we reach the core of the Launcher which runs the Guest, serves
* its input and output, and finally, lays it to rest. */
static void __attribute__((noreturn)) run_guest(int lguest_fd)
{
@@ -1643,7 +1650,7 @@
err(1, "Resetting break");
}
}
-/*
+/*L:240
* This is the end of the Launcher. The good news: we are over halfway
* through! The bad news: the most fiendish part of the code still lies ahead
* of us.
@@ -1690,8 +1697,8 @@
* device receive input from a file descriptor, we keep an fdset
* (infds) and the maximum fd number (max_infd) with the head of the
* list. We also keep a pointer to the last device. Finally, we keep
- * the next interrupt number to hand out (1: remember that 0 is used by
- * the timer). */
+ * the next interrupt number to use for devices (1: remember that 0 is
+ * used by the timer). */
FD_ZERO(&devices.infds);
devices.max_infd = -1;
devices.lastdev = NULL;
@@ -1792,8 +1799,8 @@
lguest_fd = tell_kernel(pgdir, start);
/* We fork off a child process, which wakes the Launcher whenever one
- * of the input file descriptors needs attention. Otherwise we would
- * run the Guest until it tries to output something. */
+ * of the input file descriptors needs attention. We call this the
+ * Waker, and we'll cover it in a moment. */
waker_fd = setup_waker(lguest_fd);
/* Finally, run the Guest. This doesn't return. */