lguest: documentation IV: Launcher

Documentation: The Launcher

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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 = &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)
 {
@@ -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(&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;
@@ -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 = &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;
 
@@ -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".
+ */