path: root/drivers/lguest/io.c

/*P:300 The I/O mechanism in lguest is simple yet flexible, allowing the Guest
 * to talk to the Launcher or directly to another Guest.  It uses familiar
 * concepts of DMA and interrupts, plus some neat code stolen from
 * futexes... :*/

/* Copyright (C) 2006 Rusty Russell IBM Corporation
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program; if not, write to the Free Software
 *  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 */
#include <linux/types.h>
#include <linux/futex.h>
#include <linux/jhash.h>
#include <linux/mm.h>
#include <linux/highmem.h>
#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;

	for (i = 0; i < ARRAY_SIZE(dma_hash); i++)
		INIT_LIST_HEAD(&dma_hash[i]);
}

/* FIXME: allow multi-page lengths. */
static int check_dma_list(struct lguest *lg, const struct lguest_dma *dma)
{
	unsigned int i;

	for (i = 0; i < LGUEST_MAX_DMA_SECTIONS; i++) {
		if (!dma->len[i])
			return 1;
		if (!lguest_address_ok(lg, dma->addr[i], dma->len[i]))
			goto kill;
		if (dma->len[i] > PAGE_SIZE)
			goto kill;
		/* We could do over a page, but is it worth it? */
		if ((dma->addr[i] % PAGE_SIZE) + dma->len[i] > PAGE_SIZE)
			goto kill;
	}
	return 1;

kill:
	kill_guest(lg, "bad DMA entry: %u@%#lx", dma->len[i], dma->addr[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,
		      (sizeof(key->both.word)+sizeof(key->both.ptr))/4,
		      key->both.offset)
		% 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.ptr == b->both.ptr
		&& a->both.offset == b->both.offset);
}

/*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;
			break;
		}
	}
	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(lg->mem_base + 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].owner = lg;
			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;
}

/*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)
{
	if (!lguest_address_ok(lg, addr, bytes)