From dde797899ac17ebb812b7566044124d785e98dc7 Mon Sep 17 00:00:00 2001 From: Rusty Russell Date: Thu, 26 Jul 2007 10:41:03 -0700 Subject: lguest: documentation IV: Launcher Documentation: The Launcher Signed-off-by: Rusty Russell Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/lguest/lguest_user.c | 159 ++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 150 insertions(+), 9 deletions(-) (limited to 'drivers/lguest/lguest_user.c') diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c index 6ae86f2..80d1b58 100644 --- a/drivers/lguest/lguest_user.c +++ b/drivers/lguest/lguest_user.c @@ -9,33 +9,62 @@ #include #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", -- cgit v1.1