Boot with virtual == physical to get closer to native Linux.
1) This allows us to get alot closer to booting bzImages.
2) It means we don't have to know page_offset.
3) The Guest needs to modify the boot pagetables to create the
PAGE_OFFSET mapping before jumping to C code.
4) guest_pa() walks the page tables rather than using page_offset.
5) We don't use page_offset to figure out whether to emulate: it was
always kinda quesationable, and won't work for instructions done
before remapping (bzImage unpacking in particular).
6) We still want the kernel address for tlb flushing: have the initial
hypercall give us that, too.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/Documentation/lguest/lguest.c b/Documentation/lguest/lguest.c
index 4950b03..32c2eaf 100644
--- a/Documentation/lguest/lguest.c
+++ b/Documentation/lguest/lguest.c
@@ -178,19 +178,16 @@
/* 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(const void *start, const void *end,
- unsigned long page_offset)
+static unsigned long entry_point(const void *start, const void *end)
{
const void *p;
- /* The scan gives us the physical starting address. We want the
- * virtual address in this case, and fortunately, we already figured
- * out the physical-virtual difference and passed it here in
- * "page_offset". */
+ /* The scan gives us the physical starting address. We boot with
+ * pagetables set up with virtual and physical the same, so that's
+ * OK. */
for (p = start; p < end; p++)
if (memcmp(p, "GenuineLguest", strlen("GenuineLguest")) == 0)
- return to_guest_phys(p + strlen("GenuineLguest"))
- + page_offset;
+ return to_guest_phys(p + strlen("GenuineLguest"));
errx(1, "Is this image a genuine lguest?");
}
@@ -224,14 +221,11 @@
* by all modern binaries on Linux including the kernel.
*
* The ELF headers give *two* addresses: a physical address, and a virtual
- * address. The Guest kernel expects to be placed in memory at the physical
- * address, and the page tables set up so it will correspond to that virtual
- * address. We return the difference between the virtual and physical
- * addresses in the "page_offset" pointer.
+ * address. We use the physical address; the Guest will map itself to the
+ * virtual address.
*
* We return the starting address. */
-static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr,
- unsigned long *page_offset)
+static unsigned long map_elf(int elf_fd, const Elf32_Ehdr *ehdr)
{
void *start = (void *)-1, *end = NULL;
Elf32_Phdr phdr[ehdr->e_phnum];
@@ -255,9 +249,6 @@
if (read(elf_fd, phdr, sizeof(phdr)) != sizeof(phdr))
err(1, "Reading program headers");
- /* We don't know page_offset yet. */
- *page_offset = 0;
-
/* Try all the headers: there are usually only three. A read-only one,
* a read-write one, and a "note" section which isn't loadable. */
for (i = 0; i < ehdr->e_phnum; i++) {
@@ -268,14 +259,6 @@
verbose("Section %i: size %i addr %p\n",
i, phdr[i].p_memsz, (void *)phdr[i].p_paddr);
- /* We expect a simple linear address space: every segment must
- * have the same difference between virtual (p_vaddr) and
- * physical (p_paddr) address. */
- if (!*page_offset)
- *page_offset = phdr[i].p_vaddr - phdr[i].p_paddr;
- else if (*page_offset != phdr[i].p_vaddr - phdr[i].p_paddr)
- errx(1, "Page offset of section %i different", i);
-
/* We track the first and last address we mapped, so we can
* tell entry_point() where to scan. */
if (from_guest_phys(phdr[i].p_paddr) < start)
@@ -288,50 +271,13 @@
phdr[i].p_offset, phdr[i].p_filesz);
}
- return entry_point(start, end, *page_offset);
-}
-
-/*L:170 Prepare to be SHOCKED and AMAZED. And possibly a trifle nauseated.
- *
- * We know that CONFIG_PAGE_OFFSET sets what virtual address the kernel expects
- * to be. We don't know what that option was, but we can figure it out
- * approximately by looking at the addresses in the code. I chose the common
- * case of reading a memory location into the %eax register:
- *
- * movl <some-address>, %eax
- *
- * This gets encoded as five bytes: "0xA1 <4-byte-address>". For example,
- * "0xA1 0x18 0x60 0x47 0xC0" reads the address 0xC0476018 into %eax.
- *
- * In this example can guess that the kernel was compiled with
- * CONFIG_PAGE_OFFSET set to 0xC0000000 (it's always a round number). If the
- * kernel were larger than 16MB, we might see 0xC1 addresses show up, but our
- * kernel isn't that bloated yet.
- *
- * Unfortunately, x86 has variable-length instructions, so finding this
- * particular instruction properly involves writing a disassembler. Instead,
- * we rely on statistics. We look for "0xA1" and tally the different bytes
- * which occur 4 bytes later (the "0xC0" in our example above). When one of
- * those bytes appears three times, we can be reasonably confident that it
- * forms the start of CONFIG_PAGE_OFFSET.
- *
- * This is amazingly reliable. */
-static unsigned long intuit_page_offset(unsigned char *img, unsigned long len)
-{
- unsigned int i, possibilities[256] = { 0 };
-
- for (i = 0; i + 4 < len; i++) {
- /* mov 0xXXXXXXXX,%eax */
- if (img[i] == 0xA1 && ++possibilities[img[i+4]] > 3)
- return (unsigned long)img[i+4] << 24;
- }
- errx(1, "could not determine page offset");
+ return entry_point(start, end);
}
/*L:160 Unfortunately the entire ELF image isn't compressed: the segments
* which need loading are extracted and compressed raw. This denies us the
* information we need to make a fully-general loader. */
-static unsigned long unpack_bzimage(int fd, unsigned long *page_offset)
+static unsigned long unpack_bzimage(int fd)
{
gzFile f;
int ret, len = 0;
@@ -352,12 +298,7 @@
verbose("Unpacked size %i addr %p\n", len, img);
- /* Without the ELF header, we can't tell virtual-physical gap. This is
- * CONFIG_PAGE_OFFSET, and people do actually change it. Fortunately,
- * I have a clever way of figuring it out from the code itself. */
- *page_offset = intuit_page_offset(img, len);
-
- return entry_point(img, img + len, *page_offset);
+ return entry_point(img, img + len);
}
/*L:150 A bzImage, unlike an ELF file, is not meant to be loaded. You're
@@ -368,7 +309,7 @@
* The bzImage is formed by putting the decompressing code in front of the
* compressed kernel code. So we can simple scan through it looking for the
* first "gzip" header, and start decompressing from there. */
-static unsigned long load_bzimage(int fd, unsigned long *page_offset)
+static unsigned long load_bzimage(int fd)
{
unsigned char c;
int state = 0;
@@ -396,7 +337,7 @@
if (c != 0x03)
state = -1;
else
- return unpack_bzimage(fd, page_offset);
+ return unpack_bzimage(fd);
}
}
errx(1, "Could not find kernel in bzImage");
@@ -405,7 +346,7 @@
/*L:140 Loading the kernel is easy when it's a "vmlinux", but most kernels
* come wrapped up in the self-decompressing "bzImage" format. With some funky
* coding, we can load those, too. */
-static unsigned long load_kernel(int fd, unsigned long *page_offset)
+static unsigned long load_kernel(int fd)
{
Elf32_Ehdr hdr;
@@ -415,10 +356,10 @@
/* If it's an ELF file, it starts with "\177ELF" */
if (memcmp(hdr.e_ident, ELFMAG, SELFMAG) == 0)
- return map_elf(fd, &hdr, page_offset);
+ return map_elf(fd, &hdr);
/* Otherwise we assume it's a bzImage, and try to unpack it */
- return load_bzimage(fd, page_offset);
+ return load_bzimage(fd);
}
/* This is a trivial little helper to align pages. Andi Kleen hated it because
@@ -463,27 +404,20 @@
return len;
}
-/* 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
+/* Once we know how much memory we have, we can construct simple linear page
+ * tables which set virtual == physical 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). */
static unsigned long setup_pagetables(unsigned long mem,
- unsigned long initrd_size,
- unsigned long page_offset)
+ unsigned long initrd_size)
{
unsigned long *pgdir, *linear;
unsigned int mapped_pages, i, linear_pages;
unsigned int ptes_per_page = getpagesize()/sizeof(void *);
- /* Ideally we map all physical memory starting at page_offset.
- * However, if page_offset is 0xC0000000 we can only map 1G of physical
- * (0xC0000000 + 1G overflows). */
- if (mem <= -page_offset)
- mapped_pages = mem/getpagesize();
- else
- mapped_pages = -page_offset/getpagesize();
+ mapped_pages = mem/getpagesize();
/* Each PTE page can map ptes_per_page pages: how many do we need? */
linear_pages = (mapped_pages + ptes_per_page-1)/ptes_per_page;
@@ -500,11 +434,9 @@
for (i = 0; i < mapped_pages; i++)
linear[i] = ((i * getpagesize()) | PAGE_PRESENT);
- /* The top level points to the linear page table pages above. The
- * entry representing page_offset points to the first one, and they
- * continue from there. */
+ /* The top level points to the linear page table pages above. */
for (i = 0; i < mapped_pages; i += ptes_per_page) {
- pgdir[(i + page_offset/getpagesize())/ptes_per_page]
+ pgdir[i/ptes_per_page]
= ((to_guest_phys(linear) + i*sizeof(void *))
| PAGE_PRESENT);
}
@@ -535,15 +467,12 @@
/* 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 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(unsigned long pgdir, unsigned long start,
- unsigned long page_offset)
+ * top level pagetable and the entry point for the Guest. */
+static int tell_kernel(unsigned long pgdir, unsigned long start)
{
unsigned long args[] = { LHREQ_INITIALIZE,
(unsigned long)guest_base,
- guest_limit / getpagesize(),
- pgdir, start, page_offset };
+ guest_limit / getpagesize(), pgdir, start };
int fd;
verbose("Guest: %p - %p (%#lx)\n",
@@ -1424,9 +1353,9 @@
/*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
- * of the (optional) initrd. */
- unsigned long mem = 0, pgdir, start, page_offset, initrd_size = 0;
+ /* Memory, top-level pagetable, code startpoint and size of the
+ * (optional) initrd. */
+ unsigned long mem = 0, pgdir, start, initrd_size = 0;
/* A temporary and the /dev/lguest file descriptor. */
int i, c, lguest_fd;
/* The list of Guest devices, based on command line arguments. */
@@ -1500,8 +1429,7 @@
setup_console(&device_list);
/* Now we load the kernel */
- start = load_kernel(open_or_die(argv[optind+1], O_RDONLY),
- &page_offset);
+ start = load_kernel(open_or_die(argv[optind+1], O_RDONLY));
/* Boot information is stashed at physical address 0 */
boot = from_guest_phys(0);
@@ -1518,7 +1446,7 @@
}
/* Set up the initial linear pagetables, starting below the initrd. */
- pgdir = setup_pagetables(mem, initrd_size, page_offset);
+ pgdir = setup_pagetables(mem, initrd_size);
/* The Linux boot header contains an "E820" memory map: ours is a
* simple, single region. */
@@ -1535,7 +1463,7 @@
/* We tell the kernel to initialize the Guest: this returns the open
* /dev/lguest file descriptor. */
- lguest_fd = tell_kernel(pgdir, start, page_offset);
+ lguest_fd = tell_kernel(pgdir, start);
/* We fork off a child process, which wakes the Launcher whenever one
* of the input file descriptors needs attention. Otherwise we would