-/* Simple I/O model for guests, based on shared memory.
- * Copyright (C) 2006 Rusty Russell IBM Corporation
+/*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
#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 = ¤t->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(¤t->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. */
+/*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)
{
struct lguest_dma_info *i;
struct rw_semaphore *fshared = ¤t->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".
+ */
projects / linux-2.6-omap-h63xx.git / blobdiff