[linux-2.6] / 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].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;
}

/*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)
            || access_process_vm(lg->tsk, (unsigned long)lg->mem_base + addr,
                                 buf, bytes, 0) != bytes) {
                memset(buf, 0, bytes);
                kill_guest(lg, "bad address in registered DMA struct");
                return 0;
        }
        return 1;
}

/* "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)
{
        if (!lguest_address_ok(lg, addr, bytes)
            || access_process_vm(lg->tsk, (unsigned long)lg->mem_base + addr,
                                 (void *)buf, bytes, 1) != bytes) {
                kill_guest(lg, "bad address writing to registered DMA");
                return 0;
        }
        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,
                     struct page *pages[])
{
        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]);

                /* 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,
                                   srclg->mem_base+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++;
                        dstoff = 0;
                }
        }

        /* If we still had a page mapped at the end, unmap now. */
        if (maddr)
                kunmap(pages[di]);

        return totlen;
}

/*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)
{
        int i;
        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;

        /* 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,
                                   (unsigned long)dstlg->mem_base+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 the data until we run out of src or dst. */
        ret = copy_data(srclg, src, dst, pages);

drop_pages:
        while (--i >= 0)
                put_page(pages[i]);
        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_dma dst_dma, src_dma;
        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];
        /* Read in the source "struct lguest_dma" handed to SEND_DMA. */
        lgread(srclg, &src_dma, udma, sizeof(src_dma));

        /* 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),
                                  sizeof(dst_dma))) {
                        goto fail;
                }
                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 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;

                /* 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);

        /* 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:
        up_read(&dstlg->mm->mmap_sem);
        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;
        int empty = 0;
        struct rw_semaphore *fshared = &current->mm->mmap_sem;

again:
        mutex_lock(&lguest_lock);
        down_read(fshared);
        /* Get the futex key for the key the Guest gave us */
        if (get_futex_key(lg->mem_base + ukey, fshared, &key) != 0) {
                kill_guest(lg, "bad sending DMA key");
                goto unlock;
        }
        /* 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: 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;
        }
unlock:
        up_read(fshared);
        mutex_unlock(&lguest_lock);
}
/*:*/

void release_all_dma(struct lguest *lg)
{
        unsigned int i;

        BUG_ON(!mutex_is_locked(&lguest_lock));

        down_read(&lg->mm->mmap_sem);
        for (i = 0; i < LGUEST_MAX_DMA; i++) {
                if (lg->dma[i].interrupt)
                        unlink_dma(&lg->dma[i]);
        }
        up_read(&lg->mm->mmap_sem);
}

/*M:007 We only return a single DMA buffer to the Launcher, but it would be
 * more efficient to return a pointer to the entire array of DMA buffers, which
 * it can cache and choose one whenever it wants.
 *
 * Currently the Launcher uses a write to /dev/lguest, and the return value is
 * the address of the DMA structure with the interrupt number placed in
 * dma->used_len.  If we wanted to return the entire array, we need to return
 * the address, array size and interrupt number: this seems to require an
 * ioctl(). :*/

/*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)
{
        unsigned long ret = 0;
        union futex_key key;
        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(lg->mem_base + 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;

                                ret = i->dmas + j * sizeof(struct lguest_dma);
                                lgread(lg, &dma, ret, sizeof(dma));
                                if (dma.used_len == 0)
                                        break;
                        }
                        /* Store the interrupt the Guest wants when the buffer
                         * is used. */
                        *interrupt = i->interrupt;
                        break;
                }
        }
unlock:
        up_read(fshared);
        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".
 */