diff options
Diffstat (limited to 'drivers/lguest/io.c')
| -rw-r--r-- | drivers/lguest/io.c | 247 |
1 files changed, 230 insertions, 17 deletions
diff --git a/drivers/lguest/io.c b/drivers/lguest/io.c index d2f02f0653ca..da288128e44f 100644 --- a/drivers/lguest/io.c +++ b/drivers/lguest/io.c @@ -27,8 +27,36 @@ #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; @@ -60,6 +88,19 @@ kill: 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, @@ -68,6 +109,9 @@ static unsigned int hash(const union futex_key *key) % 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 @@ -75,22 +119,36 @@ static inline int key_eq(const union futex_key *a, const union futex_key *b) && 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; @@ -100,51 +158,91 @@ static int unbind_dma(struct lguest *lg, 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) { @@ -157,7 +255,8 @@ static int lgread_other(struct lguest *lg, 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) { @@ -170,6 +269,15 @@ static int lgwrite_other(struct lguest *lg, u32 addr, 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, @@ -178,33 +286,59 @@ static u32 copy_data(struct lguest *srclg, 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++; @@ -212,13 +346,15 @@ static u32 copy_data(struct lguest *srclg, } } + /* 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) { @@ -226,23 +362,31 @@ static u32 do_dma(struct lguest *srclg, const struct lguest_dma *src, 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: @@ -251,6 +395,11 @@ 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) @@ -259,15 +408,23 @@ static int dma_transfer(struct lguest *srclg, 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), @@ -277,30 +434,46 @@ static int dma_transfer(struct lguest *srclg, 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: @@ -308,6 +481,8 @@ 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; @@ -317,31 +492,43 @@ void send_dma(struct lguest *lg, unsigned long ukey, unsigned long udma) 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; @@ -350,6 +537,7 @@ unlock: up_read(fshared); mutex_unlock(&lguest_lock); } +/*:*/ void release_all_dma(struct lguest *lg) { @@ -365,7 +553,8 @@ 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) { @@ -374,15 +563,29 @@ unsigned long get_dma_buffer(struct lguest *lg, 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; @@ -391,6 +594,8 @@ unsigned long get_dma_buffer(struct lguest *lg, if (dma.used_len == 0) break; } + /* Store the interrupt the Guest wants when the buffer + * is used. */ *interrupt = i->interrupt; break; } @@ -400,4 +605,12 @@ unlock: 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". + */ |