blob: ea68613b43f6282fb922d19a5af30b0e2073b870 [file] [log] [blame]
/*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
* 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"
* 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++)
/* 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_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,
% 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? */
/* This is how we know that the entry is free. */
dmainfo->interrupt = 0;
/* Remove it from the hash table. */
/* Drop the references we were holding (to the inode or mm). */
/*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) {
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 = &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. */
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. */
/* 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)
/* 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. */
/* Unlock and out. */
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, 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, 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,
(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;
/* 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 */
srcoff = 0;
if (dstoff == dst->len[di]) {
/* We need to unmap that destination page and reset
* maddr ready for the next one. */
maddr = NULL;
dstoff = 0;
/* If we still had a page mapped at the end, unmap now. */
if (maddr)
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)
/* 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 the data until we run out of src or dst. */
ret = copy_data(srclg, src, dst, pages);
while (--i >= 0)
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. */
/* 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)
/* 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. */
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. */
/* 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. */
/* 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. */
/* If we passed the last "struct lguest_dma", the receive had no
* buffers left. */
return i == dst->num_dmas;
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;
/* 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;
/* 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)
if (!key_eq(&key, &i->key))
/* If dma_transfer() tells us the destination has no
* available buffers, we increment "empty". */
empty += dma_transfer(lg, udma, i);
/* 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. */
/* Next time, we won't try again. */
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;
void release_all_dma(struct lguest *lg)
unsigned int i;
for (i = 0; i < LGUEST_MAX_DMA; i++) {
if (lg->dma[i].interrupt)
/*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. */
/* 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. */
/* 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;
ret = i->dmas + j * sizeof(struct lguest_dma);
lgread(lg, &dma, ret, sizeof(dma));
if (dma.used_len == 0)
/* Store the interrupt the Guest wants when the buffer
* is used. */
*interrupt = i->interrupt;
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".