diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/lguest/lguest.c | 155 |
1 files changed, 95 insertions, 60 deletions
diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c index c91c28ae829..f2668390e8f 100644 --- a/Documentation/lguest/lguest.c +++ b/Documentation/lguest/lguest.c @@ -360,8 +360,8 @@ static unsigned long load_bzimage(int fd) } /*L:140 Loading the kernel is easy when it's a "vmlinux", but most kernels - * come wrapped up in the self-decompressing "bzImage" format. With some funky - * coding, we can load those, too. */ + * come wrapped up in the self-decompressing "bzImage" format. With a little + * work, we can load those, too. */ static unsigned long load_kernel(int fd) { Elf32_Ehdr hdr; @@ -464,6 +464,7 @@ static unsigned long setup_pagetables(unsigned long mem, * to know where it is. */ return to_guest_phys(pgdir); } +/*:*/ /* Simple routine to roll all the commandline arguments together with spaces * between them. */ @@ -480,9 +481,9 @@ static void concat(char *dst, char *args[]) dst[len] = '\0'; } -/* This is where we actually tell the kernel to initialize the Guest. We saw - * the arguments it expects when we looked at initialize() in lguest_user.c: - * the base of guest "physical" memory, the top physical page to allow, the +/*L:185 This is where we actually tell the kernel to initialize the Guest. We + * saw the arguments it expects when we looked at initialize() in lguest_user.c: + * the base of Guest "physical" memory, the top physical page to allow, the * top level pagetable and the entry point for the Guest. */ static int tell_kernel(unsigned long pgdir, unsigned long start) { @@ -512,13 +513,14 @@ static void add_device_fd(int fd) /*L:200 * The Waker. * - * With a console and network devices, we can have lots of input which we need - * to process. We could try to tell the kernel what file descriptors to watch, - * but handing a file descriptor mask through to the kernel is fairly icky. + * With console, block and network devices, we can have lots of input which we + * need to process. We could try to tell the kernel what file descriptors to + * watch, but handing a file descriptor mask through to the kernel is fairly + * icky. * * Instead, we fork off a process which watches the file descriptors and writes - * the LHREQ_BREAK command to the /dev/lguest filedescriptor to tell the Host - * loop to stop running the Guest. This causes it to return from the + * the LHREQ_BREAK command to the /dev/lguest file descriptor to tell the Host + * stop running the Guest. This causes the Launcher to return from the * /dev/lguest read with -EAGAIN, where it will write to /dev/lguest to reset * the LHREQ_BREAK and wake us up again. * @@ -544,7 +546,9 @@ static void wake_parent(int pipefd, int lguest_fd) if (read(pipefd, &fd, sizeof(fd)) == 0) exit(0); /* Otherwise it's telling us to change what file - * descriptors we're to listen to. */ + * descriptors we're to listen to. Positive means + * listen to a new one, negative means stop + * listening. */ if (fd >= 0) FD_SET(fd, &devices.infds); else @@ -559,7 +563,7 @@ static int setup_waker(int lguest_fd) { int pipefd[2], child; - /* We create a pipe to talk to the waker, and also so it knows when the + /* We create a pipe to talk to the Waker, and also so it knows when the * Launcher dies (and closes pipe). */ pipe(pipefd); child = fork(); @@ -567,7 +571,8 @@ static int setup_waker(int lguest_fd) err(1, "forking"); if (child == 0) { - /* Close the "writing" end of our copy of the pipe */ + /* We are the Waker: close the "writing" end of our copy of the + * pipe and start waiting for input. */ close(pipefd[1]); wake_parent(pipefd[0], lguest_fd); } @@ -578,12 +583,12 @@ static int setup_waker(int lguest_fd) return pipefd[1]; } -/*L:210 +/* * Device Handling. * - * When the Guest sends DMA to us, it sends us an array of addresses and sizes. + * When the Guest gives us a buffer, it sends an array of addresses and sizes. * We need to make sure it's not trying to reach into the Launcher itself, so - * we have a convenient routine which check it and exits with an error message + * we have a convenient routine which checks it and exits with an error message * if something funny is going on: */ static void *_check_pointer(unsigned long addr, unsigned int size, @@ -600,7 +605,9 @@ static void *_check_pointer(unsigned long addr, unsigned int size, /* A macro which transparently hands the line number to the real function. */ #define check_pointer(addr,size) _check_pointer(addr, size, __LINE__) -/* This function returns the next descriptor in the chain, or vq->vring.num. */ +/* Each buffer in the virtqueues is actually a chain of descriptors. This + * function returns the next descriptor in the chain, or vq->vring.num if we're + * at the end. */ static unsigned next_desc(struct virtqueue *vq, unsigned int i) { unsigned int next; @@ -679,13 +686,14 @@ static unsigned get_vq_desc(struct virtqueue *vq, return head; } -/* Once we've used one of their buffers, we tell them about it. We'll then +/* After we've used one of their buffers, we tell them about it. We'll then * want to send them an interrupt, using trigger_irq(). */ static void add_used(struct virtqueue *vq, unsigned int head, int len) { struct vring_used_elem *used; - /* Get a pointer to the next entry in the used ring. */ + /* The virtqueue contains a ring of used buffers. Get a pointer to the + * next entry in that used ring. */ used = &vq->vring.used->ring[vq->vring.used->idx % vq->vring.num]; used->id = head; used->len = len; @@ -699,6 +707,7 @@ static void trigger_irq(int fd, struct virtqueue *vq) { unsigned long buf[] = { LHREQ_IRQ, vq->config.irq }; + /* If they don't want an interrupt, don't send one. */ if (vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT) return; @@ -715,8 +724,11 @@ static void add_used_and_trigger(int fd, struct virtqueue *vq, trigger_irq(fd, vq); } -/* Here is the input terminal setting we save, and the routine to restore them - * on exit so the user can see what they type next. */ +/* + * The Console + * + * Here is the input terminal setting we save, and the routine to restore them + * on exit so the user gets their terminal back. */ static struct termios orig_term; static void restore_term(void) { @@ -817,7 +829,10 @@ static void handle_console_output(int fd, struct virtqueue *vq) } } -/* Handling output for network is also simple: we get all the output buffers +/* + * The Network + * + * 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). */ static void handle_net_output(int fd, struct virtqueue *vq) @@ -830,8 +845,9 @@ static void handle_net_output(int fd, struct virtqueue *vq) while ((head = get_vq_desc(vq, iov, &out, &in)) != vq->vring.num) { if (in) errx(1, "Input buffers in output queue?"); - /* Check header, but otherwise ignore it (we said we supported - * no features). */ + /* Check header, but otherwise ignore it (we told the Guest we + * supported no features, so it shouldn't have anything + * interesting). */ (void)convert(&iov[0], struct virtio_net_hdr); len = writev(vq->dev->fd, iov+1, out-1); add_used_and_trigger(fd, vq, head, len); @@ -882,7 +898,8 @@ static bool handle_tun_input(int fd, struct device *dev) return true; } -/* This callback ensures we try again, in case we stopped console or net +/*L:215 This is the callback attached to the network and console input + * virtqueues: it ensures we try again, in case we stopped console or net * delivery because Guest didn't have any buffers. */ static void enable_fd(int fd, struct virtqueue *vq) { @@ -918,7 +935,7 @@ static void handle_output(int fd, unsigned long addr) strnlen(from_guest_phys(addr), guest_limit - addr)); } -/* This is called when the waker wakes us up: check for incoming file +/* This is called when the Waker wakes us up: check for incoming file * descriptors. */ static void handle_input(int fd) { @@ -985,8 +1002,7 @@ static struct lguest_device_desc *new_dev_desc(u16 type) } /* Each device descriptor is followed by some configuration information. - * The first byte is a "status" byte for the Guest to report what's happening. - * After that are fields: u8 type, u8 len, [... len bytes...]. + * Each configuration field looks like: u8 type, u8 len, [... len bytes...]. * * This routine adds a new field to an existing device's descriptor. It only * works for the last device, but that's OK because that's how we use it. */ @@ -1043,14 +1059,17 @@ static void add_virtqueue(struct device *dev, unsigned int num_descs, /* Link virtqueue back to device. */ vq->dev = dev; - /* Set up handler. */ + /* Set the routine to call when the Guest does something to this + * virtqueue. */ vq->handle_output = handle_output; + + /* Set the "Don't Notify Me" flag if we don't have a handler */ if (!handle_output) vq->vring.used->flags = VRING_USED_F_NO_NOTIFY; } /* This routine does all the creation and setup of a new device, including - * caling new_dev_desc() to allocate the descriptor and device memory. */ + * calling new_dev_desc() to allocate the descriptor and device memory. */ static struct device *new_device(const char *name, u16 type, int fd, bool (*handle_input)(int, struct device *)) { @@ -1059,7 +1078,7 @@ static struct device *new_device(const char *name, u16 type, int fd, /* Append to device list. Prepending to a single-linked list is * easier, but the user expects the devices to be arranged on the bus * in command-line order. The first network device on the command line - * is eth0, the first block device /dev/lgba, etc. */ + * is eth0, the first block device /dev/vda, etc. */ *devices.lastdev = dev; dev->next = NULL; devices.lastdev = &dev->next; @@ -1103,7 +1122,7 @@ static void setup_console(void) /* The console needs two virtqueues: the input then the output. When * they put something the input queue, we make sure we're listening to * stdin. When they put something in the output queue, we write it to - * stdout. */ + * stdout. */ add_virtqueue(dev, VIRTQUEUE_NUM, enable_fd); add_virtqueue(dev, VIRTQUEUE_NUM, handle_console_output); @@ -1251,21 +1270,17 @@ static void setup_tun_net(const char *arg) verbose("attached to bridge: %s\n", br_name); } - -/* - * Block device. +/* Our block (disk) device should be really simple: the Guest asks for a block + * number and we read or write that position in the file. Unfortunately, that + * was amazingly slow: the Guest waits until the read is finished before + * running anything else, even if it could have been doing useful work. * - * Serving a block device is really easy: the Guest asks for a block number and - * we read or write that position in the file. - * - * Unfortunately, this is amazingly slow: the Guest waits until the read is - * finished before running anything else, even if it could be doing useful - * work. We could use async I/O, except it's reputed to suck so hard that - * characters actually go missing from your code when you try to use it. + * We could use async I/O, except it's reputed to suck so hard that characters + * actually go missing from your code when you try to use it. * * So we farm the I/O out to thread, and communicate with it via a pipe. */ -/* This hangs off device->priv, with the data. */ +/* This hangs off device->priv. */ struct vblk_info { /* The size of the file. */ @@ -1281,8 +1296,14 @@ struct vblk_info * Launcher triggers interrupt to Guest. */ int done_fd; }; +/*:*/ -/* This is the core of the I/O thread. It returns true if it did something. */ +/*L:210 + * The Disk + * + * Remember that the block device is handled by a separate I/O thread. We head + * straight into the core of that thread here: + */ static bool service_io(struct device *dev) { struct vblk_info *vblk = dev->priv; @@ -1293,10 +1314,14 @@ static bool service_io(struct device *dev) struct iovec iov[dev->vq->vring.num]; off64_t off; + /* See if there's a request waiting. If not, nothing to do. */ head = get_vq_desc(dev->vq, iov, &out_num, &in_num); if (head == dev->vq->vring.num) return false; + /* Every block request should contain at least one output buffer + * (detailing the location on disk and the type of request) and one + * input buffer (to hold the result). */ if (out_num == 0 || in_num == 0) errx(1, "Bad virtblk cmd %u out=%u in=%u", head, out_num, in_num); @@ -1305,10 +1330,15 @@ static bool service_io(struct device *dev) in = convert(&iov[out_num+in_num-1], struct virtio_blk_inhdr); off = out->sector * 512; - /* This is how we implement barriers. Pretty poor, no? */ + /* The block device implements "barriers", where the Guest indicates + * that it wants all previous writes to occur before this write. We + * don't have a way of asking our kernel to do a barrier, so we just + * synchronize all the data in the file. Pretty poor, no? */ if (out->type & VIRTIO_BLK_T_BARRIER) fdatasync(vblk->fd); + /* In general the virtio block driver is allowed to try SCSI commands. + * It'd be nice if we supported eject, for example, but we don't. */ if (out->type & VIRTIO_BLK_T_SCSI_CMD) { fprintf(stderr, "Scsi commands unsupported\n"); in->status = VIRTIO_BLK_S_UNSUPP; @@ -1374,7 +1404,7 @@ static int io_thread(void *_dev) /* When this read fails, it means Launcher died, so we follow. */ while (read(vblk->workpipe[0], &c, 1) == 1) { - /* We acknowledge each request immediately, to reduce latency, + /* 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. */ while (service_io(dev)) @@ -1383,12 +1413,14 @@ static int io_thread(void *_dev) return 0; } -/* When the thread says some I/O is done, we interrupt the Guest. */ +/* 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. */ static bool handle_io_finish(int fd, struct device *dev) { char c; - /* If child died, presumably it printed message. */ + /* If the I/O thread died, presumably it printed the error, so we + * simply exit. */ if (read(dev->fd, &c, 1) != 1) exit(1); @@ -1397,7 +1429,7 @@ static bool handle_io_finish(int fd, struct device *dev) return true; } -/* When the Guest submits some I/O, we wake the I/O thread. */ +/* When the Guest submits some I/O, we just need to wake the I/O thread. */ static void handle_virtblk_output(int fd, struct virtqueue *vq) { struct vblk_info *vblk = vq->dev->priv; @@ -1409,7 +1441,7 @@ static void handle_virtblk_output(int fd, struct virtqueue *vq) exit(1); } -/* This creates a virtual block device. */ +/*L:198 This actually sets up a virtual block device. */ static void setup_block_file(const char *filename) { int p[2]; @@ -1425,7 +1457,7 @@ static void setup_block_file(const char *filename) /* The device responds to return from I/O thread. */ dev = new_device("block", VIRTIO_ID_BLOCK, p[0], handle_io_finish); - /* The device has a virtqueue. */ + /* The device has one virtqueue, where the Guest places requests. */ add_virtqueue(dev, VIRTQUEUE_NUM, handle_virtblk_output); /* Allocate the room for our own bookkeeping */ @@ -1447,7 +1479,8 @@ static void setup_block_file(const char *filename) /* The I/O thread writes to this end of the pipe when done. */ vblk->done_fd = p[1]; - /* This is how we tell the I/O thread about more work. */ + /* This is the second pipe, which is how we tell the I/O thread about + * more work. */ pipe(vblk->workpipe); /* Create stack for thread and run it */ @@ -1486,24 +1519,25 @@ static void __attribute__((noreturn)) run_guest(int lguest_fd) char reason[1024] = { 0 }; read(lguest_fd, reason, sizeof(reason)-1); errx(1, "%s", reason); - /* EAGAIN means the waker wanted us to look at some input. + /* EAGAIN means the Waker wanted us to look at some input. * Anything else means a bug or incompatible change. */ } else if (errno != EAGAIN) err(1, "Running guest failed"); - /* Service input, then unset the BREAK which releases - * the Waker. */ + /* Service input, then unset the BREAK to release the Waker. */ handle_input(lguest_fd); if (write(lguest_fd, args, sizeof(args)) < 0) err(1, "Resetting break"); } } /* - * This is the end of the Launcher. + * 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. * - * But wait! We've seen I/O from the Launcher, and we've seen I/O from the - * Drivers. If we were to see the Host kernel I/O code, our understanding - * would be complete... :*/ + * Are you ready? Take a deep breath and join me in the core of the Host, in + * "make Host". + :*/ static struct option opts[] = { { "verbose", 0, NULL, 'v' }, @@ -1526,7 +1560,7 @@ int main(int argc, char *argv[]) /* Memory, top-level pagetable, code startpoint and size of the * (optional) initrd. */ unsigned long mem = 0, pgdir, start, initrd_size = 0; - /* A temporary and the /dev/lguest file descriptor. */ + /* Two temporaries and the /dev/lguest file descriptor. */ int i, c, lguest_fd; /* The boot information for the Guest. */ struct boot_params *boot; @@ -1621,6 +1655,7 @@ int main(int argc, char *argv[]) /* The boot header contains a command line pointer: we put the command * line after the boot header. */ boot->hdr.cmd_line_ptr = to_guest_phys(boot + 1); + /* We use a simple helper to copy the arguments separated by spaces. */ concat((char *)(boot + 1), argv+optind+2); /* Boot protocol version: 2.07 supports the fields for lguest. */ |