diff options
Diffstat (limited to 'Documentation/lguest')
| -rw-r--r-- | Documentation/lguest/lguest.c | 69 |
1 files changed, 38 insertions, 31 deletions
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 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr) 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 @@ static unsigned long load_kernel(int fd) 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 @@ static unsigned long load_initrd(const char *name, unsigned long mem) 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 @@ static void handle_console_output(int fd, struct virtqueue *vq) * * 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 @@ static void enable_fd(int fd, struct virtqueue *vq) 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 @@ static void handle_input(int fd) 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 @@ static void handle_input(int fd) * 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 @@ static void handle_input(int fd) * * 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 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs, 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 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs, } /* 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 @@ static void set_config(struct device *dev, unsigned len, const void *conf) } /* 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 @@ static int io_thread(void *_dev) 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 @@ static int io_thread(void *_dev) } /* 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 @@ static void setup_block_file(const char *filename) * 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 @@ static void setup_block_file(const char *filename) 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 @@ static void __attribute__((noreturn)) restart_guest(void) 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 @@ static void __attribute__((noreturn)) run_guest(int lguest_fd) 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 @@ int main(int argc, char *argv[]) * 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 @@ int main(int argc, char *argv[]) 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. */ |