lguest: documentation IV: Launcher

	return 1;

}

/*L:305

 * Dealing With Guest Memory.

 *

 * When the Guest gives us (what it thinks is) a physical address, we can use

 * the normal copy_from_user() & copy_to_user() on that address: remember,

 * Guest physical == Launcher virtual.

 *

 * But we can't trust the Guest: it might be trying to access the Launcher

 * code.  We have to check that the range is below the pfn_limit the Launcher

 * gave us.  We have to make sure that addr + len doesn't give us a false

 * positive by overflowing, too. */

int lguest_address_ok(const struct lguest *lg,

		      unsigned long addr, unsigned long len)

{

	return (addr+len) / PAGE_SIZE < lg->pfn_limit && (addr+len >= addr);

}

/* Just like get_user, but don't let guest access lguest binary. */

/* This is a convenient routine to get a 32-bit value from the Guest (a very

 * common operation).  Here we can see how useful the kill_lguest() routine we

 * met in the Launcher can be: we return a random value (0) instead of needing

 * to return an error. */

u32 lgread_u32(struct lguest *lg, unsigned long addr)

{

	u32 val = 0;

	/* Don't let them access lguest binary */

	/* Don't let them access lguest binary. */

	if (!lguest_address_ok(lg, addr, sizeof(val))

	    || get_user(val, (u32 __user *)addr) != 0)

		kill_guest(lg, "bad read address %#lx", addr);

	return val;

}

/* Same thing for writing a value. */

void lgwrite_u32(struct lguest *lg, unsigned long addr, u32 val)

{

	if (!lguest_address_ok(lg, addr, sizeof(val))

		kill_guest(lg, "bad write address %#lx", addr);

}

/* This routine is more generic, and copies a range of Guest bytes into a

 * buffer.  If the copy_from_user() fails, we fill the buffer with zeroes, so

 * the caller doesn't end up using uninitialized kernel memory. */

void lgread(struct lguest *lg, void *b, unsigned long addr, unsigned bytes)

{

	if (!lguest_address_ok(lg, addr, bytes)

	}

}

/* Similarly, our generic routine to copy into a range of Guest bytes. */

void lgwrite(struct lguest *lg, unsigned long addr, const void *b,

	     unsigned bytes)

{

	    || copy_to_user((void __user *)addr, b, bytes) != 0)

		kill_guest(lg, "bad write address %#lx len %u", addr, bytes);

}

/* (end of memory access helper routines) :*/

static void set_ts(void)

{

#include <linux/uaccess.h>

#include "lg.h"

/*L:300

 * I/O

 *

 * Getting data in and out of the Guest is quite an art.  There are numerous

 * ways to do it, and they all suck differently.  We try to keep things fairly

 * close to "real" hardware so our Guest's drivers don't look like an alien

 * visitation in the middle of the Linux code, and yet make sure that Guests

 * can talk directly to other Guests, not just the Launcher.

 *

 * To do this, the Guest gives us a key when it binds or sends DMA buffers.

 * The key corresponds to a "physical" address inside the Guest (ie. a virtual

 * address inside the Launcher process).  We don't, however, use this key

 * directly.

 *

 * We want Guests which share memory to be able to DMA to each other: two

 * Launchers can mmap memory the same file, then the Guests can communicate.

 * Fortunately, the futex code provides us with a way to get a "union

 * futex_key" corresponding to the memory lying at a virtual address: if the

 * two processes share memory, the "union futex_key" for that memory will match

 * even if the memory is mapped at different addresses in each.  So we always

 * convert the keys to "union futex_key"s to compare them.

 *

 * Before we dive into this though, we need to look at another set of helper

 * routines used throughout the Host kernel code to access Guest memory.

 :*/

static struct list_head dma_hash[61];

/* An unfortunate side effect of the Linux double-linked list implementation is

 * that there's no good way to statically initialize an array of linked

 * lists. */

void lguest_io_init(void)

{

	unsigned int i;

	return 0;

}

/*L:330 This is our hash function, using the wonderful Jenkins hash.

 *

 * The futex key is a union with three parts: an unsigned long word, a pointer,

 * and an int "offset".  We could use jhash_2words() which takes three u32s.

 * (Ok, the hash functions are great: the naming sucks though).

 *

 * It's nice to be portable to 64-bit platforms, so we use the more generic

 * jhash2(), which takes an array of u32, the number of u32s, and an initial

 * u32 to roll in.  This is uglier, but breaks down to almost the same code on

 * 32-bit platforms like this one.

 *

 * We want a position in the array, so we modulo ARRAY_SIZE(dma_hash) (ie. 61).

 */

static unsigned int hash(const union futex_key *key)

{

	return jhash2((u32*)&key->both.word,

		% ARRAY_SIZE(dma_hash);

}

/* This is a convenience routine to compare two keys.  It's a much bemoaned C

 * weakness that it doesn't allow '==' on structures or unions, so we have to

 * open-code it like this. */

static inline int key_eq(const union futex_key *a, const union futex_key *b)

{

	return (a->both.word == b->both.word

		&& a->both.offset == b->both.offset);

}

/* Must hold read lock on dmainfo owner's current->mm->mmap_sem */

/*L:360 OK, when we need to actually free up a Guest's DMA array we do several

 * things, so we have a convenient function to do it.

 *

 * The caller must hold a read lock on dmainfo owner's current->mm->mmap_sem

 * for the drop_futex_key_refs(). */

static void unlink_dma(struct lguest_dma_info *dmainfo)

{

	/* You locked this too, right? */

	BUG_ON(!mutex_is_locked(&lguest_lock));

	/* This is how we know that the entry is free. */

	dmainfo->interrupt = 0;

	/* Remove it from the hash table. */

	list_del(&dmainfo->list);

	/* Drop the references we were holding (to the inode or mm). */

	drop_futex_key_refs(&dmainfo->key);

}

/*L:350 This is the routine which we call when the Guest asks to unregister a

 * DMA array attached to a given key.  Returns true if the array was found. */

static int unbind_dma(struct lguest *lg,

		      const union futex_key *key,

		      unsigned long dmas)

{

	int i, ret = 0;

	/* We don't bother with the hash table, just look through all this

	 * Guest's DMA arrays. */

	for (i = 0; i < LGUEST_MAX_DMA; i++) {

		/* In theory it could have more than one array on the same key,

		 * or one array on multiple keys, so we check both */

		if (key_eq(key, &lg->dma[i].key) && dmas == lg->dma[i].dmas) {

			unlink_dma(&lg->dma[i]);

			ret = 1;

	return ret;

}

/*L:340 BIND_DMA: this is the hypercall which sets up an array of "struct

 * lguest_dma" for receiving I/O.

 *

 * The Guest wants to bind an array of "struct lguest_dma"s to a particular key

 * to receive input.  This only happens when the Guest is setting up a new

 * device, so it doesn't have to be very fast.

 *

 * It returns 1 on a successful registration (it can fail if we hit the limit

 * of registrations for this Guest).

 */

int bind_dma(struct lguest *lg,

	     unsigned long ukey, unsigned long dmas, u16 numdmas, u8 interrupt)

{

	unsigned int i;

	int ret = 0;

	union futex_key key;

	/* Futex code needs the mmap_sem. */

	struct rw_semaphore *fshared = &current->mm->mmap_sem;

	/* Invalid interrupt?  (We could kill the guest here). */

	if (interrupt >= LGUEST_IRQS)

		return 0;

	/* We need to grab the Big Lguest Lock, because other Guests may be

	 * trying to look through this Guest's DMAs to send something while

	 * we're doing this. */

	mutex_lock(&lguest_lock);

	down_read(fshared);

	if (get_futex_key((u32 __user *)ukey, fshared, &key) != 0) {

		kill_guest(lg, "bad dma key %#lx", ukey);

		goto unlock;

	}

	/* We want to keep this key valid once we drop mmap_sem, so we have to

	 * hold a reference. */

	get_futex_key_refs(&key);

	/* If the Guest specified an interrupt of 0, that means they want to

	 * unregister this array of "struct lguest_dma"s. */

	if (interrupt == 0)

		ret = unbind_dma(lg, &key, dmas);

	else {

		/* Look through this Guest's dma array for an unused entry. */

		for (i = 0; i < LGUEST_MAX_DMA; i++) {

			/* If the interrupt is non-zero, the entry is already

			 * used. */

			if (lg->dma[i].interrupt)

				continue;

			/* OK, a free one!  Fill on our details. */

			lg->dma[i].dmas = dmas;

			lg->dma[i].num_dmas = numdmas;

			lg->dma[i].next_dma = 0;

			lg->dma[i].key = key;

			lg->dma[i].guestid = lg->guestid;

			lg->dma[i].interrupt = interrupt;

			/* Now we add it to the hash table: the position

			 * depends on the futex key that we got. */

			list_add(&lg->dma[i].list, &dma_hash[hash(&key)]);

			/* Success! */

			ret = 1;

			goto unlock;

		}

	}

	/* If we didn't find a slot to put the key in, drop the reference

	 * again. */

	drop_futex_key_refs(&key);

unlock:

	/* Unlock and out. */

 	up_read(fshared);

	mutex_unlock(&lguest_lock);

	return ret;

}

/* lgread from another guest */

/*L:385 Note that our routines to access a different Guest's memory are called

 * lgread_other() and lgwrite_other(): these names emphasize that they are only

 * used when the Guest is *not* the current Guest.

 *

 * The interface for copying from another process's memory is called

 * access_process_vm(), with a final argument of 0 for a read, and 1 for a

 * write.

 *

 * We need lgread_other() to read the destination Guest's "struct lguest_dma"

 * array. */

static int lgread_other(struct lguest *lg,

			void *buf, u32 addr, unsigned bytes)

{

	return 1;

}

/* lgwrite to another guest */

/* "lgwrite()" to another Guest: used to update the destination "used_len" once

 * we've transferred data into the buffer. */

static int lgwrite_other(struct lguest *lg, u32 addr,

			 const void *buf, unsigned bytes)

{

	return 1;

}

/*L:400 This is the generic engine which copies from a source "struct

 * lguest_dma" from this Guest into another Guest's "struct lguest_dma".  The

 * destination Guest's pages have already been mapped, as contained in the

 * pages array.

 *

 * If you're wondering if there's a nice "copy from one process to another"

 * routine, so was I.  But Linux isn't really set up to copy between two

 * unrelated processes, so we have to write it ourselves.

 */

static u32 copy_data(struct lguest *srclg,

		     const struct lguest_dma *src,

		     const struct lguest_dma *dst,

	unsigned int totlen, si, di, srcoff, dstoff;

	void *maddr = NULL;

	/* We return the total length transferred. */

	totlen = 0;

	/* We keep indexes into the source and destination "struct lguest_dma",

	 * and an offset within each region. */

	si = di = 0;

	srcoff = dstoff = 0;

	/* We loop until the source or destination is exhausted. */

	while (si < LGUEST_MAX_DMA_SECTIONS && src->len[si]

	       && di < LGUEST_MAX_DMA_SECTIONS && dst->len[di]) {

		/* We can only transfer the rest of the src buffer, or as much

		 * as will fit into the destination buffer. */

		u32 len = min(src->len[si] - srcoff, dst->len[di] - dstoff);

		/* For systems using "highmem" we need to use kmap() to access

		 * the page we want.  We often use the same page over and over,

		 * so rather than kmap() it on every loop, we set the maddr

		 * pointer to NULL when we need to move to the next

		 * destination page. */

		if (!maddr)

			maddr = kmap(pages[di]);

		/* FIXME: This is not completely portable, since

		   archs do different things for copy_to_user_page. */

		/* Copy directly from (this Guest's) source address to the

		 * destination Guest's kmap()ed buffer.  Note that maddr points

		 * to the start of the page: we need to add the offset of the

		 * destination address and offset within the buffer. */

		/* FIXME: This is not completely portable.  I looked at

		 * copy_to_user_page(), and some arch's seem to need special

		 * flushes.  x86 is fine. */

		if (copy_from_user(maddr + (dst->addr[di] + dstoff)%PAGE_SIZE,

				   (void __user *)src->addr[si], len) != 0) {

			/* If a copy failed, it's the source's fault. */

			kill_guest(srclg, "bad address in sending DMA");

			totlen = 0;

			break;

		}

		/* Increment the total and src & dst offsets */

		totlen += len;

		srcoff += len;

		dstoff += len;

		/* Presumably we reached the end of the src or dest buffers: */

		if (srcoff == src->len[si]) {

			/* Move to the next buffer at offset 0 */

			si++;

			srcoff = 0;

		}

		if (dstoff == dst->len[di]) {

			/* We need to unmap that destination page and reset

			 * maddr ready for the next one. */

			kunmap(pages[di]);

			maddr = NULL;

			di++;

		}

	}

	/* If we still had a page mapped at the end, unmap now. */

	if (maddr)

		kunmap(pages[di]);

	return totlen;

}

/* Src is us, ie. current. */

/*L:390 This is how we transfer a "struct lguest_dma" from the source Guest

 * (the current Guest which called SEND_DMA) to another Guest. */

static u32 do_dma(struct lguest *srclg, const struct lguest_dma *src,

		  struct lguest *dstlg, const struct lguest_dma *dst)

{

	u32 ret;

	struct page *pages[LGUEST_MAX_DMA_SECTIONS];

	/* We check that both source and destination "struct lguest_dma"s are

	 * within the bounds of the source and destination Guests */

	if (!check_dma_list(dstlg, dst) || !check_dma_list(srclg, src))

		return 0;

	/* First get the destination pages */

	/* We need to map the pages which correspond to each parts of

	 * destination buffer. */

	for (i = 0; i < LGUEST_MAX_DMA_SECTIONS; i++) {

		if (dst->len[i] == 0)

			break;

		/* get_user_pages() is a complicated function, especially since

		 * we only want a single page.  But it works, and returns the

		 * number of pages.  Note that we're holding the destination's

		 * mmap_sem, as get_user_pages() requires. */

		if (get_user_pages(dstlg->tsk, dstlg->mm,

				   dst->addr[i], 1, 1, 1, pages+i, NULL)

		    != 1) {

			/* This means the destination gave us a bogus buffer */

			kill_guest(dstlg, "Error mapping DMA pages");

			ret = 0;

			goto drop_pages;

		}

	}

	/* Now copy until we run out of src or dst. */

	/* Now copy the data until we run out of src or dst. */

	ret = copy_data(srclg, src, dst, pages);

drop_pages:

	return ret;

}

/*L:380 Transferring data from one Guest to another is not as simple as I'd

 * like.  We've found the "struct lguest_dma_info" bound to the same address as

 * the send, we need to copy into it.

 *

 * This function returns true if the destination array was empty. */

static int dma_transfer(struct lguest *srclg,

			unsigned long udma,

			struct lguest_dma_info *dst)

	struct lguest *dstlg;

	u32 i, dma = 0;

	/* From the "struct lguest_dma_info" we found in the hash, grab the

	 * Guest. */

	dstlg = &lguests[dst->guestid];

	/* Get our dma list. */

	/* Read in the source "struct lguest_dma" handed to SEND_DMA. */

	lgread(srclg, &src_dma, udma, sizeof(src_dma));

	/* We can't deadlock against them dmaing to us, because this

	 * is all under the lguest_lock. */

	/* We need the destination's mmap_sem, and we already hold the source's

	 * mmap_sem for the futex key lookup.  Normally this would suggest that

	 * we could deadlock if the destination Guest was trying to send to

	 * this source Guest at the same time, which is another reason that all

	 * I/O is done under the big lguest_lock. */

	down_read(&dstlg->mm->mmap_sem);

	/* Look through the destination DMA array for an available buffer. */

	for (i = 0; i < dst->num_dmas; i++) {

		/* We keep a "next_dma" pointer which often helps us avoid

		 * looking at lots of previously-filled entries. */

		dma = (dst->next_dma + i) % dst->num_dmas;

		if (!lgread_other(dstlg, &dst_dma,

				  dst->dmas + dma * sizeof(struct lguest_dma),

		if (!dst_dma.used_len)

			break;

	}

	/* If we found a buffer, we do the actual data copy. */

	if (i != dst->num_dmas) {

		unsigned long used_lenp;

		unsigned int ret;

		ret = do_dma(srclg, &src_dma, dstlg, &dst_dma);

		/* Put used length in src. */

		/* Put used length in the source "struct lguest_dma"'s used_len

		 * field.  It's a little tricky to figure out where that is,

		 * though. */

		lgwrite_u32(srclg,

			    udma+offsetof(struct lguest_dma, used_len), ret);

		/* Tranferring 0 bytes is OK if the source buffer was empty. */

		if (ret == 0 && src_dma.len[0] != 0)

			goto fail;

		/* Make sure destination sees contents before length. */

		/* The destination Guest might be running on a different CPU:

		 * we have to make sure that it will see the "used_len" field

		 * change to non-zero *after* it sees the data we copied into

		 * the buffer.  Hence a write memory barrier. */

		wmb();

		/* Figuring out where the destination's used_len field for this

		 * "struct lguest_dma" in the array is also a little ugly. */

		used_lenp = dst->dmas

			+ dma * sizeof(struct lguest_dma)

			+ offsetof(struct lguest_dma, used_len);

		lgwrite_other(dstlg, used_lenp, &ret, sizeof(ret));

		/* Move the cursor for next time. */

		dst->next_dma++;

	}

 	up_read(&dstlg->mm->mmap_sem);

	/* Do this last so dst doesn't simply sleep on lock. */

	/* We trigger the destination interrupt, even if the destination was

	 * empty and we didn't transfer anything: this gives them a chance to

	 * wake up and refill. */

	set_bit(dst->interrupt, dstlg->irqs_pending);

	/* Wake up the destination process. */

	wake_up_process(dstlg->tsk);

	/* If we passed the last "struct lguest_dma", the receive had no

	 * buffers left. */

	return i == dst->num_dmas;

fail:

	return 0;

}

/*L:370 This is the counter-side to the BIND_DMA hypercall; the SEND_DMA

 * hypercall.  We find out who's listening, and send to them. */

void send_dma(struct lguest *lg, unsigned long ukey, unsigned long udma)

{

	union futex_key key;

again:

	mutex_lock(&lguest_lock);

	down_read(fshared);

	/* Get the futex key for the key the Guest gave us */

	if (get_futex_key((u32 __user *)ukey, fshared, &key) != 0) {

		kill_guest(lg, "bad sending DMA key");

		goto unlock;

	}

	/* Shared mapping?  Look for other guests... */

	/* Since the key must be a multiple of 4, the futex key uses the lower

	 * bit of the "offset" field (which would always be 0) to indicate a

	 * mapping which is shared with other processes (ie. Guests). */

	if (key.shared.offset & 1) {

		struct lguest_dma_info *i;

		/* Look through the hash for other Guests. */

		list_for_each_entry(i, &dma_hash[hash(&key)], list) {

			/* Don't send to ourselves. */

			if (i->guestid == lg->guestid)

				continue;

			if (!key_eq(&key, &i->key))

				continue;

			/* If dma_transfer() tells us the destination has no

			 * available buffers, we increment "empty". */

			empty += dma_transfer(lg, udma, i);

			break;

		}

		/* If the destination is empty, we release our locks and

		 * give the destination Guest a brief chance to restock. */

		if (empty == 1) {

			/* Give any recipients one chance to restock. */

			up_read(&current->mm->mmap_sem);

			mutex_unlock(&lguest_lock);

			/* Next time, we won't try again. */

			empty++;

			goto again;

		}

	} else {

		/* Private mapping: tell our userspace. */

		/* Private mapping: Guest is sending to its Launcher.  We set

		 * the "dma_is_pending" flag so that the main loop will exit

		 * and the Launcher's read() from /dev/lguest will return. */

		lg->dma_is_pending = 1;

		lg->pending_dma = udma;

		lg->pending_key = ukey;

	up_read(fshared);

	mutex_unlock(&lguest_lock);

}

/*:*/

void release_all_dma(struct lguest *lg)

{

	up_read(&lg->mm->mmap_sem);

}

/* Userspace wants a dma buffer from this guest. */

/*L:320 This routine looks for a DMA buffer registered by the Guest on the

 * given key (using the BIND_DMA hypercall). */

unsigned long get_dma_buffer(struct lguest *lg,

			     unsigned long ukey, unsigned long *interrupt)

{

	struct lguest_dma_info *i;

	struct rw_semaphore *fshared = &current->mm->mmap_sem;

	/* Take the Big Lguest Lock to stop other Guests sending this Guest DMA

	 * at the same time. */

	mutex_lock(&lguest_lock);

	/* To match between Guests sharing the same underlying memory we steal

	 * code from the futex infrastructure.  This requires that we hold the

	 * "mmap_sem" for our process (the Launcher), and pass it to the futex

	 * code. */

	down_read(fshared);

	/* This can fail if it's not a valid address, or if the address is not

	 * divisible by 4 (the futex code needs that, we don't really). */

	if (get_futex_key((u32 __user *)ukey, fshared, &key) != 0) {

		kill_guest(lg, "bad registered DMA buffer");

		goto unlock;

	}

	/* Search the hash table for matching entries (the Launcher can only

	 * send to its own Guest for the moment, so the entry must be for this

	 * Guest) */

	list_for_each_entry(i, &dma_hash[hash(&key)], list) {

		if (key_eq(&key, &i->key) && i->guestid == lg->guestid) {

			unsigned int j;

			/* Look through the registered DMA array for an

			 * available buffer. */

			for (j = 0; j < i->num_dmas; j++) {

				struct lguest_dma dma;

				if (dma.used_len == 0)

					break;

			}

			/* Store the interrupt the Guest wants when the buffer

			 * is used. */

			*interrupt = i->interrupt;

			break;

		}

	mutex_unlock(&lguest_lock);

	return ret;

}

/*:*/

/*L:410 This really has completed the Launcher.  Not only have we now finished

 * the longest chapter in our journey, but this also means we are over halfway

 * through!

 *

 * Enough prevaricating around the bush: it is time for us to dive into the

 * core of the Host, in "make Host".

 */

/* hypercalls.c: */

void do_hypercalls(struct lguest *lg);

/*L:035

 * Let's step aside for the moment, to study one important routine that's used

 * widely in the Host code.

 *

 * There are many cases where the Guest does something invalid, like pass crap

 * to a hypercall.  Since only the Guest kernel can make hypercalls, it's quite

 * acceptable to simply terminate the Guest and give the Launcher a nicely

 * formatted reason.  It's also simpler for the Guest itself, which doesn't

 * need to check most hypercalls for "success"; if you're still running, it

 * succeeded.

 *

 * Once this is called, the Guest will never run again, so most Host code can

 * call this then continue as if nothing had happened.  This means many

 * functions don't have to explicitly return an error code, which keeps the

 * code simple.

 *

 * It also means that this can be called more than once: only the first one is

 * remembered.  The only trick is that we still need to kill the Guest even if

 * we can't allocate memory to store the reason.  Linux has a neat way of

 * packing error codes into invalid pointers, so we use that here.

 *

 * Like any macro which uses an "if", it is safely wrapped in a run-once "do {

 * } while(0)".

 */

#define kill_guest(lg, fmt...)					\

do {								\

	if (!(lg)->dead) {					\

			(lg)->dead = ERR_PTR(-ENOMEM);		\

	}							\

} while(0)

/* (End of aside) :*/

static inline unsigned long guest_pa(struct lguest *lg, unsigned long vaddr)

{