Introduce guest mem offset, static link example launcher

In order to avoid problematic special linking of the Launcher, we give
the Host an offset: this means we can use any memory region in the
Launcher as Guest memory rather than insisting on mmap() at 0.

The result is quite pleasing: a number of casts are replaced with
simple additions.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

8 files changed, 163 insertions, 140 deletions

diff --git a/Documentation/lguest/Makefile b/Documentation/lguest/Makefile
index 526c15fd83af..bac037eb1cda 100644
--- a/Documentation/lguest/Makefile
+++ b/Documentation/lguest/Makefile
@@ -1,28 +1,8 @@
 # This creates the demonstration utility "lguest" which runs a Linux guest.
-
-# For those people that have a separate object dir, look there for .config
-KBUILD_OUTPUT := ../..
-ifdef O
-  ifeq ("$(origin O)", "command line")
-    KBUILD_OUTPUT := $(O)
-  endif
-endif
-# We rely on CONFIG_PAGE_OFFSET to know where to put lguest binary.
-include $(KBUILD_OUTPUT)/.config
-LGUEST_GUEST_TOP := ($(CONFIG_PAGE_OFFSET) - 0x08000000)
-
-CFLAGS:=-Wall -Wmissing-declarations -Wmissing-prototypes -O3 -I../../include -Wl,-T,lguest.lds
+CFLAGS:=-Wall -Wmissing-declarations -Wmissing-prototypes -O3 -I../../include
 LDLIBS:=-lz
-# Removing this works for some versions of ld.so (eg. Ubuntu Feisty) and
-# not others (eg. FC7).
-LDFLAGS+=-static
-all: lguest.lds lguest
 
-# The linker script on x86 is so complex the only way of creating one
-# which will link our binary in the right place is to mangle the
-# default one.
-lguest.lds:
-        $(LD) --verbose | awk '/^==========/ { PRINT=1; next; } /SIZEOF_HEADERS/ { gsub(/0x[0-9A-F]*/, "$(LGUEST_GUEST_TOP)") } { if (PRINT) print $$0; }' > $@
+all: lguest
 
 clean:
-        rm -f lguest.lds lguest
+        rm -f lguest
diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index 401d26b464ff..140bd98a8417 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -1,10 +1,7 @@
 /*P:100 This is the Launcher code, a simple program which lays out the
  * "physical" memory for the new Guest by mapping the kernel image and the
  * virtual devices, then reads repeatedly from /dev/lguest to run the Guest.
- *
- * The only trick: the Makefile links it at a high address so it will be clear
- * of the guest memory region.  It means that each Guest cannot have more than
- * about 2.5G of memory on a normally configured Host. :*/
+:*/
 #define _LARGEFILE64_SOURCE
 #define _GNU_SOURCE
 #include <stdio.h>
@@ -56,6 +53,8 @@ typedef uint8_t u8;
 #ifndef SIOCBRADDIF
 #define SIOCBRADDIF     0x89a2          /* add interface to bridge      */
 #endif
+/* We can have up to 256 pages for devices. */
+#define DEVICE_PAGES 256
 
 /*L:120 verbose is both a global flag and a macro.  The C preprocessor allows
  * this, and although I wouldn't recommend it, it works quite nicely here. */
@@ -66,8 +65,10 @@ static bool verbose;
 
 /* The pipe to send commands to the waker process */
 static int waker_fd;
-/* The top of guest physical memory. */
-static u32 top;
+/* The pointer to the start of guest memory. */
+static void *guest_base;
+/* The maximum guest physical address allowed, and maximum possible. */
+static unsigned long guest_limit, guest_max;
 
 /* This is our list of devices. */
 struct device_list
@@ -111,6 +112,29 @@ struct device
         void *priv;
 };
 
+/*L:100 The Launcher code itself takes us out into userspace, that scary place
+ * where pointers run wild and free!  Unfortunately, like most userspace
+ * programs, it's quite boring (which is why everyone likes to hack on the
+ * kernel!).  Perhaps if you make up an Lguest Drinking Game at this point, it
+ * will get you through this section.  Or, maybe not.
+ *
+ * The Launcher sets up a big chunk of memory to be the Guest's "physical"
+ * memory and stores it in "guest_base".  In other words, Guest physical ==
+ * Launcher virtual with an offset.
+ *
+ * This can be tough to get your head around, but usually it just means that we
+ * use these trivial conversion functions when the Guest gives us it's
+ * "physical" addresses: */
+static void *from_guest_phys(unsigned long addr)
+{
+        return guest_base + addr;
+}
+
+static unsigned long to_guest_phys(const void *addr)
+{
+        return (addr - guest_base);
+}
+
 /*L:130
  * Loading the Kernel.
  *
@@ -124,33 +148,40 @@ static int open_or_die(const char *name, int flags)
         return fd;
 }
 
-/* map_zeroed_pages() takes a (page-aligned) address and a number of pages. */
-static void *map_zeroed_pages(unsigned long addr, unsigned int num)
+/* map_zeroed_pages() takes a number of pages. */
+static void *map_zeroed_pages(unsigned int num)
 {
-        /* We cache the /dev/zero file-descriptor so we only open it once. */
-        static int fd = -1;
-
-        if (fd == -1)
-                fd = open_or_die("/dev/zero", O_RDONLY);
+        int fd = open_or_die("/dev/zero", O_RDONLY);
+        void *addr;
 
         /* We use a private mapping (ie. if we write to the page, it will be
-         * copied), and obviously we insist that it be mapped where we ask. */
-        if (mmap((void *)addr, getpagesize() * num,
-                 PROT_READ|PROT_WRITE|PROT_EXEC, MAP_FIXED|MAP_PRIVATE, fd, 0)
-            != (void *)addr)
-                err(1, "Mmaping %u pages of /dev/zero @%p", num, (void *)addr);
-
-        /* Returning the address is just a courtesy: can simplify callers. */
-        return (void *)addr;
+         * copied). */
+        addr = mmap(NULL, getpagesize() * num,
+                    PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE, fd, 0);
+        if (addr == MAP_FAILED)
+                err(1, "Mmaping %u pages of /dev/zero", num);
+
+        return addr;
+}
+
+/* Get some more pages for a device. */
+static void *get_pages(unsigned int num)
+{
+        void *addr = from_guest_phys(guest_limit);
+
+        guest_limit += num * getpagesize();
+        if (guest_limit > guest_max)
+                errx(1, "Not enough memory for devices");
+        return addr;
 }
 
 /* To find out where to start we look for the magic Guest string, which marks
  * the code we see in lguest_asm.S.  This is a hack which we are currently
  * plotting to replace with the normal Linux entry point. */
-static unsigned long entry_point(void *start, void *end,
+static unsigned long entry_point(const void *start, const void *end,
                                  unsigned long page_offset)
 {
-        void *p;
+        const void *p;
 
         /* The scan gives us the physical starting address.  We want the
          * virtual address in this case, and fortunately, we already figured
@@ -158,7 +189,8 @@ static unsigned long entry_point(void *start, void *end,
          * "page_offset". */
         for (p = start; p < end; p++)
                 if (memcmp(p, "GenuineLguest", strlen("GenuineLguest")) == 0)
-                        return (long)p + strlen("GenuineLguest") + page_offset;
+                        return to_guest_phys(p + strlen("GenuineLguest"))
+                                + page_offset;
 
         errx(1, "Is this image a genuine lguest?");
 }
@@ -201,9 +233,9 @@ static void map_at(int fd, void *addr, unsigned long offset, unsigned long len)
 static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
                              unsigned long *page_offset)
 {
+        void *start = (void *)-1, *end = NULL;
         Elf32_Phdr phdr[ehdr->e_phnum];
         unsigned int i;
-        unsigned long start = -1UL, end = 0;
 
         /* Sanity checks on the main ELF header: an x86 executable with a
          * reasonable number of correctly-sized program headers. */
@@ -246,17 +278,17 @@ static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
 
                 /* We track the first and last address we mapped, so we can
                  * tell entry_point() where to scan. */
-                if (phdr[i].p_paddr < start)
-                        start = phdr[i].p_paddr;
-                if (phdr[i].p_paddr + phdr[i].p_filesz > end)
-                        end = phdr[i].p_paddr + phdr[i].p_filesz;
+                if (from_guest_phys(phdr[i].p_paddr) < start)
+                        start = from_guest_phys(phdr[i].p_paddr);
+                if (from_guest_phys(phdr[i].p_paddr) + phdr[i].p_filesz > end)
+                        end=from_guest_phys(phdr[i].p_paddr)+phdr[i].p_filesz;
 
                 /* We map this section of the file at its physical address. */
-                map_at(elf_fd, (void *)phdr[i].p_paddr,
+                map_at(elf_fd, from_guest_phys(phdr[i].p_paddr),
                        phdr[i].p_offset, phdr[i].p_filesz);
         }
 
-        return entry_point((void *)start, (void *)end, *page_offset);
+        return entry_point(start, end, *page_offset);
 }
 
 /*L:170 Prepare to be SHOCKED and AMAZED.  And possibly a trifle nauseated.
@@ -307,7 +339,7 @@ static unsigned long unpack_bzimage(int fd, unsigned long *page_offset)
          * actually configurable as CONFIG_PHYSICAL_START, but as the comment
          * there says, "Don't change this unless you know what you are doing".
          * Indeed. */
-        void *img = (void *)0x100000;
+        void *img = from_guest_phys(0x100000);
 
         /* gzdopen takes our file descriptor (carefully placed at the start of
          * the GZIP header we found) and returns a gzFile. */
@@ -421,7 +453,7 @@ static unsigned long load_initrd(const char *name, unsigned long mem)
         /* We map the initrd at the top of memory, but mmap wants it to be
          * page-aligned, so we round the size up for that. */
         len = page_align(st.st_size);
-        map_at(ifd, (void *)mem - len, 0, st.st_size);
+        map_at(ifd, from_guest_phys(mem - len), 0, st.st_size);
         /* Once a file is mapped, you can close the file descriptor.  It's a
          * little odd, but quite useful. */
         close(ifd);
@@ -431,9 +463,9 @@ static unsigned long load_initrd(const char *name, unsigned long mem)
         return len;
 }
 
-/* Once we know how much memory we have, and the address the Guest kernel
- * expects, we can construct simple linear page tables which will get the Guest
- * far enough into the boot to create its own.
+/* Once we know the address the Guest kernel expects, we can construct simple
+ * linear page tables for all of memory which will get the Guest far enough
+ * into the boot to create its own.
  *
  * We lay them out of the way, just below the initrd (which is why we need to
  * know its size). */
@@ -457,7 +489,7 @@ static unsigned long setup_pagetables(unsigned long mem,
         linear_pages = (mapped_pages + ptes_per_page-1)/ptes_per_page;
 
         /* We put the toplevel page directory page at the top of memory. */
-        pgdir = (void *)mem - initrd_size - getpagesize();
+        pgdir = from_guest_phys(mem) - initrd_size - getpagesize();
 
         /* Now we use the next linear_pages pages as pte pages */
         linear = (void *)pgdir - linear_pages*getpagesize();
@@ -473,15 +505,16 @@ static unsigned long setup_pagetables(unsigned long mem,
          * continue from there. */
         for (i = 0; i < mapped_pages; i += ptes_per_page) {
                 pgdir[(i + page_offset/getpagesize())/ptes_per_page]
-                        = (((u32)linear + i*sizeof(u32)) | PAGE_PRESENT);
+                        = ((to_guest_phys(linear) + i*sizeof(u32))
+                           | PAGE_PRESENT);
         }
 
-        verbose("Linear mapping of %u pages in %u pte pages at %p\n",
-                mapped_pages, linear_pages, linear);
+        verbose("Linear mapping of %u pages in %u pte pages at %#lx\n",
+                mapped_pages, linear_pages, to_guest_phys(linear));
 
         /* We return the top level (guest-physical) address: the kernel needs
          * to know where it is. */
-        return (unsigned long)pgdir;
+        return to_guest_phys(pgdir);
 }
 
 /* Simple routine to roll all the commandline arguments together with spaces
@@ -501,14 +534,19 @@ static void concat(char *dst, char *args[])
 
 /* This is where we actually tell the kernel to initialize the Guest.  We saw
  * the arguments it expects when we looked at initialize() in lguest_user.c:
- * the top physical page to allow, the top level pagetable, the entry point and
- * the page_offset constant for the Guest. */
+ * the base of guest "physical" memory, the top physical page to allow, the
+ * top level pagetable, the entry point and the page_offset constant for the
+ * Guest. */
 static int tell_kernel(u32 pgdir, u32 start, u32 page_offset)
 {
         u32 args[] = { LHREQ_INITIALIZE,
-                       top/getpagesize(), pgdir, start, page_offset };
+                       (unsigned long)guest_base,
+                       guest_limit / getpagesize(),
+                       pgdir, start, page_offset };
         int fd;
 
+        verbose("Guest: %p - %p (%#lx)\n",
+                guest_base, guest_base + guest_limit, guest_limit);
         fd = open_or_die("/dev/lguest", O_RDWR);
         if (write(fd, args, sizeof(args)) < 0)
                 err(1, "Writing to /dev/lguest");
@@ -605,11 +643,11 @@ static void *_check_pointer(unsigned long addr, unsigned int size,
 {
         /* We have to separately check addr and addr+size, because size could
          * be huge and addr + size might wrap around. */
-        if (addr >= top || addr + size >= top)
+        if (addr >= guest_limit || addr + size >= guest_limit)
                 errx(1, "%s:%i: Invalid address %li", __FILE__, line, addr);
         /* We return a pointer for the caller's convenience, now we know it's
          * safe to use. */
-        return (void *)addr;
+        return from_guest_phys(addr);
 }
 /* A macro which transparently hands the line number to the real function. */
 #define check_pointer(addr,size) _check_pointer(addr, size, __LINE__)
@@ -646,7 +684,7 @@ static u32 *dma2iov(unsigned long dma, struct iovec iov[], unsigned *num)
 static u32 *get_dma_buffer(int fd, void *key,
                            struct iovec iov[], unsigned int *num, u32 *irq)
 {
-        u32 buf[] = { LHREQ_GETDMA, (u32)key };
+        u32 buf[] = { LHREQ_GETDMA, to_guest_phys(key) };
         unsigned long udma;
         u32 *res;
 
@@ -998,11 +1036,11 @@ new_dev_desc(struct lguest_device_desc *descs,
                         descs[i].features = features;
                         descs[i].num_pages = num_pages;
                         /* If they said the device needs memory, we allocate
-                         * that now, bumping up the top of Guest memory. */
+                         * that now. */
                         if (num_pages) {
-                                map_zeroed_pages(top, num_pages);
-                                descs[i].pfn = top/getpagesize();
-                                top += num_pages*getpagesize();
+                                unsigned long pa;
+                                pa = to_guest_phys(get_pages(num_pages));
+                                descs[i].pfn = pa / getpagesize();
                         }
                         return &descs[i];
                 }
@@ -1040,9 +1078,9 @@ static struct device *new_device(struct device_list *devices,
         if (handle_input)
                 set_fd(dev->fd, devices);
         dev->desc = new_dev_desc(devices->descs, type, features, num_pages);
-        dev->mem = (void *)(dev->desc->pfn * getpagesize());
+        dev->mem = from_guest_phys(dev->desc->pfn * getpagesize());
         dev->handle_input = handle_input;
-        dev->watch_key = (unsigned long)dev->mem + watch_off;
+        dev->watch_key = to_guest_phys(dev->mem) + watch_off;
         dev->handle_output = handle_output;
         return dev;
 }
@@ -1382,21 +1420,7 @@ static void usage(void)
              "<mem-in-mb> vmlinux [args...]");
 }
 
-/*L:100 The Launcher code itself takes us out into userspace, that scary place
- * where pointers run wild and free!  Unfortunately, like most userspace
- * programs, it's quite boring (which is why everyone like to hack on the
- * kernel!).  Perhaps if you make up an Lguest Drinking Game at this point, it
- * will get you through this section.  Or, maybe not.
- *
- * The Launcher binary sits up high, usually starting at address 0xB8000000.
- * Everything below this is the "physical" memory for the Guest.  For example,
- * if the Guest were to write a "1" at physical address 0, we would see a "1"
- * in the Launcher at "(int *)0".  Guest physical == Launcher virtual.
- *
- * This can be tough to get your head around, but usually it just means that we
- * don't need to do any conversion when the Guest gives us it's "physical"
- * addresses.
- */
+/*L:105 The main routine is where the real work begins: */
 int main(int argc, char *argv[])
 {
         /* Memory, top-level pagetable, code startpoint, PAGE_OFFSET and size
@@ -1406,8 +1430,8 @@ int main(int argc, char *argv[])
         int i, c, lguest_fd;
         /* The list of Guest devices, based on command line arguments. */
         struct device_list device_list;
-        /* The boot information for the Guest: at guest-physical address 0. */
-        void *boot = (void *)0;
+        /* The boot information for the Guest. */
+        void *boot;
         /* If they specify an initrd file to load. */
         const char *initrd_name = NULL;
 
@@ -1427,9 +1451,16 @@ int main(int argc, char *argv[])
          * of memory now. */
         for (i = 1; i < argc; i++) {
                 if (argv[i][0] != '-') {
-                        mem = top = atoi(argv[i]) * 1024 * 1024;
-                        device_list.descs = map_zeroed_pages(top, 1);
-                        top += getpagesize();
+                        mem = atoi(argv[i]) * 1024 * 1024;
+                        /* We start by mapping anonymous pages over all of
+                         * guest-physical memory range.  This fills it with 0,
+                         * and ensures that the Guest won't be killed when it
+                         * tries to access it. */
+                        guest_base = map_zeroed_pages(mem / getpagesize()
+                                                      + DEVICE_PAGES);
+                        guest_limit = mem;
+                        guest_max = mem + DEVICE_PAGES*getpagesize();
+                        device_list.descs = get_pages(1);
                         break;
                 }
         }
@@ -1462,18 +1493,18 @@ int main(int argc, char *argv[])
         if (optind + 2 > argc)
                 usage();
 
+        verbose("Guest base is at %p\n", guest_base);
+
         /* We always have a console device */
         setup_console(&device_list);
 
-        /* We start by mapping anonymous pages over all of guest-physical
-         * memory range.  This fills it with 0, and ensures that the Guest
-         * won't be killed when it tries to access it. */
-        map_zeroed_pages(0, mem / getpagesize());
-
         /* Now we load the kernel */
         start = load_kernel(open_or_die(argv[optind+1], O_RDONLY),
                             &page_offset);
 
+        /* Boot information is stashed at physical address 0 */
+        boot = from_guest_phys(0);
+
         /* Map the initrd image if requested (at top of physical memory) */
         if (initrd_name) {
                 initrd_size = load_initrd(initrd_name, mem);
@@ -1495,7 +1526,7 @@ int main(int argc, char *argv[])
                 = ((struct e820entry) { 0, mem, E820_RAM });
         /* The boot header contains a command line pointer: we put the command
          * line after the boot header (at address 4096) */
-        *(void **)(boot + 0x228) = boot + 4096;
+        *(u32 *)(boot + 0x228) = 4096;
         concat(boot + 4096, argv+optind+2);
 
         /* The guest type value of "1" tells the Guest it's under lguest. */
diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index a0788c12b392..eb95860cf098 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -325,8 +325,8 @@ static int emulate_insn(struct lguest *lg)
  * Dealing With Guest Memory.
  *
  * When the Guest gives us (what it thinks is) a physical address, we can use
- * the normal copy_from_user() & copy_to_user() on that address: remember,
- * Guest physical == Launcher virtual.
+ * the normal copy_from_user() & copy_to_user() on the corresponding place in
+ * the memory region allocated by the Launcher.
  *
  * But we can't trust the Guest: it might be trying to access the Launcher
  * code.  We have to check that the range is below the pfn_limit the Launcher
@@ -348,8 +348,8 @@ u32 lgread_u32(struct lguest *lg, unsigned long addr)
 
         /* Don't let them access lguest binary. */
         if (!lguest_address_ok(lg, addr, sizeof(val))
-            || get_user(val, (u32 __user *)addr) != 0)
-                kill_guest(lg, "bad read address %#lx", addr);
+            || get_user(val, (u32 *)(lg->mem_base + addr)) != 0)
+                kill_guest(lg, "bad read address %#lx: pfn_limit=%u membase=%p", addr, lg->pfn_limit, lg->mem_base);
         return val;
 }
 
@@ -357,7 +357,7 @@ u32 lgread_u32(struct lguest *lg, unsigned long addr)
 void lgwrite_u32(struct lguest *lg, unsigned long addr, u32 val)
 {
         if (!lguest_address_ok(lg, addr, sizeof(val))
-            || put_user(val, (u32 __user *)addr) != 0)
+            || put_user(val, (u32 *)(lg->mem_base + addr)) != 0)
                 kill_guest(lg, "bad write address %#lx", addr);
 }
 
@@ -367,7 +367,7 @@ void lgwrite_u32(struct lguest *lg, unsigned long addr, u32 val)
 void lgread(struct lguest *lg, void *b, unsigned long addr, unsigned bytes)
 {
         if (!lguest_address_ok(lg, addr, bytes)
-            || copy_from_user(b, (void __user *)addr, bytes) != 0) {
+            || copy_from_user(b, lg->mem_base + addr, bytes) != 0) {
                 /* copy_from_user should do this, but as we rely on it... */
                 memset(b, 0, bytes);
                 kill_guest(lg, "bad read address %#lx len %u", addr, bytes);
@@ -379,7 +379,7 @@ void lgwrite(struct lguest *lg, unsigned long addr, const void *b,
              unsigned bytes)
 {
         if (!lguest_address_ok(lg, addr, bytes)
-            || copy_to_user((void __user *)addr, b, bytes) != 0)
+            || copy_to_user(lg->mem_base + addr, b, bytes) != 0)
                 kill_guest(lg, "bad write address %#lx len %u", addr, bytes);
 }
 /* (end of memory access helper routines) :*/
@@ -616,11 +616,9 @@ int run_guest(struct lguest *lg, unsigned long __user *user)
                          *
                          * Note that if the Guest were really messed up, this
                          * could happen before it's done the INITIALIZE
-                         * hypercall, so lg->lguest_data will be NULL, so
-                         * &lg->lguest_data->cr2 will be address 8.  Writing
-                         * into that address won't hurt the Host at all,
-                         * though. */
-                        if (put_user(cr2, &lg->lguest_data->cr2))
+                         * hypercall, so lg->lguest_data will be NULL */
+                        if (lg->lguest_data
+                            && put_user(cr2, &lg->lguest_data->cr2))
                                 kill_guest(lg, "Writing cr2");
                         break;
                 case 7: /* We've intercepted a Device Not Available fault. */
diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index 5ecd60b54201..02e67b49ea4f 100644
--- a/drivers/lguest/hypercalls.c
+++ b/drivers/lguest/hypercalls.c
@@ -205,16 +205,19 @@ static void initialize(struct lguest *lg)
                 tsc_speed = 0;
 
         /* The pointer to the Guest's "struct lguest_data" is the only
-         * argument. */
-        lg->lguest_data = (struct lguest_data __user *)lg->regs->edx;
-        /* If we check the address they gave is OK now, we can simply
-         * copy_to_user/from_user from now on rather than using lgread/lgwrite.
-         * I put this in to show that I'm not immune to writing stupid
-         * optimizations. */
+         * argument.  We check that address now. */
         if (!lguest_address_ok(lg, lg->regs->edx, sizeof(*lg->lguest_data))) {
                 kill_guest(lg, "bad guest page %p", lg->lguest_data);
                 return;
         }
+
+        /* Having checked it, we simply set lg->lguest_data to point straight
+         * into the Launcher's memory at the right place and then use
+         * copy_to_user/from_user from now on, instead of lgread/write.  I put
+         * this in to show that I'm not immune to writing stupid
+         * optimizations. */
+        lg->lguest_data = lg->mem_base + lg->regs->edx;
+
         /* The Guest tells us where we're not to deliver interrupts by putting
          * the range of addresses into "struct lguest_data". */
         if (get_user(lg->noirq_start, &lg->lguest_data->noirq_start)
diff --git a/drivers/lguest/io.c b/drivers/lguest/io.c
index ea68613b43f6..3a845335fee8 100644
--- a/drivers/lguest/io.c
+++ b/drivers/lguest/io.c
@@ -186,7 +186,7 @@ int bind_dma(struct lguest *lg,
          * we're doing this. */
         mutex_lock(&lguest_lock);
         down_read(fshared);
-        if (get_futex_key((u32 __user *)ukey, fshared, &key) != 0) {
+        if (get_futex_key(lg->mem_base + ukey, fshared, &key) != 0) {
                 kill_guest(lg, "bad dma key %#lx", ukey);
                 goto unlock;
         }
@@ -247,7 +247,8 @@ 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) {
+            || 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;
@@ -261,8 +262,8 @@ 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)) {
+            || 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;
         }
@@ -318,7 +319,7 @@ static u32 copy_data(struct lguest *srclg,
                  * 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) {
+                                   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;
@@ -377,7 +378,8 @@ static u32 do_dma(struct lguest *srclg, const struct lguest_dma *src,
                  * 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)
+                                   (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");
@@ -493,7 +495,7 @@ 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) {
+        if (get_futex_key(lg->mem_base + ukey, fshared, &key) != 0) {
                 kill_guest(lg, "bad sending DMA key");
                 goto unlock;
         }
@@ -584,7 +586,7 @@ unsigned long get_dma_buffer(struct lguest *lg,
 
         /* 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) {
+        if (get_futex_key(lg->mem_base + ukey, fshared, &key) != 0) {
                 kill_guest(lg, "bad registered DMA buffer");
                 goto unlock;
         }
diff --git a/drivers/lguest/lg.h b/drivers/lguest/lg.h
index 399eab852ab5..54f2c2472bec 100644
--- a/drivers/lguest/lg.h
+++ b/drivers/lguest/lg.h
@@ -142,6 +142,9 @@ struct lguest
         struct mm_struct *mm;   /* == tsk->mm, but that becomes NULL on exit */
         u16 guestid;
         u32 pfn_limit;
+        /* This provides the offset to the base of guest-physical
+         * memory in the Launcher. */
+        void __user *mem_base;
         u32 page_offset;
         u32 cr2;
         int halted;
diff --git a/drivers/lguest/lguest_user.c b/drivers/lguest/lguest_user.c
index 80d1b58c7698..816d4d12a801 100644
--- a/drivers/lguest/lguest_user.c
+++ b/drivers/lguest/lguest_user.c
@@ -1,9 +1,9 @@
 /*P:200 This contains all the /dev/lguest code, whereby the userspace launcher
  * controls and communicates with the Guest.  For example, the first write will
- * tell us the memory size, pagetable, entry point and kernel address offset.
- * A read will run the Guest until a signal is pending (-EINTR), or the Guest
- * does a DMA out to the Launcher.  Writes are also used to get a DMA buffer
- * registered by the Guest and to send the Guest an interrupt. :*/
+ * tell us the Guest's memory layout, pagetable, entry point and kernel address
+ * offset.  A read will run the Guest until something happens, such as a signal
+ * or the Guest doing a DMA out to the Launcher.  Writes are also used to get a
+ * DMA buffer registered by the Guest and to send the Guest an interrupt. :*/
 #include <linux/uaccess.h>
 #include <linux/miscdevice.h>
 #include <linux/fs.h>
@@ -142,9 +142,11 @@ static ssize_t read(struct file *file, char __user *user, size_t size,loff_t*o)
         return run_guest(lg, (unsigned long __user *)user);
 }
 
-/*L:020 The initialization write supplies 4 32-bit values (in addition to the
+/*L:020 The initialization write supplies 5 32-bit values (in addition to the
  * 32-bit LHREQ_INITIALIZE value).  These are:
  *
+ * base: The start of the Guest-physical memory inside the Launcher memory.
+ *
  * pfnlimit: The highest (Guest-physical) page number the Guest should be
  * allowed to access.  The Launcher has to live in Guest memory, so it sets
  * this to ensure the Guest can't reach it.
@@ -166,7 +168,7 @@ static int initialize(struct file *file, const u32 __user *input)
          * Guest. */
         struct lguest *lg;
         int err, i;
-        u32 args[4];
+        u32 args[5];
 
         /* We grab the Big Lguest lock, which protects the global array
          * "lguests" and multiple simultaneous initializations. */
@@ -194,8 +196,9 @@ static int initialize(struct file *file, const u32 __user *input)
 
         /* Populate the easy fields of our "struct lguest" */
         lg->guestid = i;
-        lg->pfn_limit = args[0];
-        lg->page_offset = args[3];
+        lg->mem_base = (void __user *)(long)args[0];
+        lg->pfn_limit = args[1];
+        lg->page_offset = args[4];
 
         /* We need a complete page for the Guest registers: they are accessible
          * to the Guest and we can only grant it access to whole pages. */
@@ -210,13 +213,13 @@ static int initialize(struct file *file, const u32 __user *input)
         /* Initialize the Guest's shadow page tables, using the toplevel
          * address the Launcher gave us.  This allocates memory, so can
          * fail. */
-        err = init_guest_pagetable(lg, args[1]);
+        err = init_guest_pagetable(lg, args[2]);
         if (err)
                 goto free_regs;
 
         /* Now we initialize the Guest's registers, handing it the start
          * address. */
-        setup_regs(lg->regs, args[2]);
+        setup_regs(lg->regs, args[3]);
 
         /* There are a couple of GDT entries the Guest expects when first
          * booting. */
diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c
index b7a924ace684..9cd2faceb87c 100644
--- a/drivers/lguest/page_tables.c
+++ b/drivers/lguest/page_tables.c
@@ -152,7 +152,7 @@ static unsigned long get_pfn(unsigned long virtpfn, int write)
 static spte_t gpte_to_spte(struct lguest *lg, gpte_t gpte, int write)
 {
         spte_t spte;
-        unsigned long pfn;
+        unsigned long pfn, base;
 
         /* The Guest sets the global flag, because it thinks that it is using
          * PGE.  We only told it to use PGE so it would tell us whether it was
@@ -160,11 +160,14 @@ static spte_t gpte_to_spte(struct lguest *lg, gpte_t gpte, int write)
          * use the global bit, so throw it away. */
         spte.flags = (gpte.flags & ~_PAGE_GLOBAL);
 
+        /* The Guest's pages are offset inside the Launcher. */
+        base = (unsigned long)lg->mem_base / PAGE_SIZE;
+
         /* We need a temporary "unsigned long" variable to hold the answer from
          * get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't
          * fit in spte.pfn.  get_pfn() finds the real physical number of the
          * page, given the virtual number. */
-        pfn = get_pfn(gpte.pfn, write);
+        pfn = get_pfn(base + gpte.pfn, write);
         if (pfn == -1UL) {
                 kill_guest(lg, "failed to get page %u", gpte.pfn);
                 /* When we destroy the Guest, we'll go through the shadow page