lguest: documentation V: Host
Documentation: The Host
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>
diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index 1eb05f9..c0f50b4 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -64,11 +64,33 @@
(SWITCHER_ADDR + SHARED_SWITCHER_PAGES*PAGE_SIZE))[cpu]);
}
+/*H:010 We need to set up the Switcher at a high virtual address. Remember the
+ * Switcher is a few hundred bytes of assembler code which actually changes the
+ * CPU to run the Guest, and then changes back to the Host when a trap or
+ * interrupt happens.
+ *
+ * The Switcher code must be at the same virtual address in the Guest as the
+ * Host since it will be running as the switchover occurs.
+ *
+ * Trying to map memory at a particular address is an unusual thing to do, so
+ * it's not a simple one-liner. We also set up the per-cpu parts of the
+ * Switcher here.
+ */
static __init int map_switcher(void)
{
int i, err;
struct page **pagep;
+ /*
+ * Map the Switcher in to high memory.
+ *
+ * It turns out that if we choose the address 0xFFC00000 (4MB under the
+ * top virtual address), it makes setting up the page tables really
+ * easy.
+ */
+
+ /* We allocate an array of "struct page"s. map_vm_area() wants the
+ * pages in this form, rather than just an array of pointers. */
switcher_page = kmalloc(sizeof(switcher_page[0])*TOTAL_SWITCHER_PAGES,
GFP_KERNEL);
if (!switcher_page) {
@@ -76,6 +98,8 @@
goto out;
}
+ /* Now we actually allocate the pages. The Guest will see these pages,
+ * so we make sure they're zeroed. */
for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) {
unsigned long addr = get_zeroed_page(GFP_KERNEL);
if (!addr) {
@@ -85,6 +109,9 @@
switcher_page[i] = virt_to_page(addr);
}
+ /* Now we reserve the "virtual memory area" we want: 0xFFC00000
+ * (SWITCHER_ADDR). We might not get it in theory, but in practice
+ * it's worked so far. */
switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE,
VM_ALLOC, SWITCHER_ADDR, VMALLOC_END);
if (!switcher_vma) {
@@ -93,49 +120,105 @@
goto free_pages;
}
+ /* This code actually sets up the pages we've allocated to appear at
+ * SWITCHER_ADDR. map_vm_area() takes the vma we allocated above, the
+ * kind of pages we're mapping (kernel pages), and a pointer to our
+ * array of struct pages. It increments that pointer, but we don't
+ * care. */
pagep = switcher_page;
err = map_vm_area(switcher_vma, PAGE_KERNEL, &pagep);
if (err) {
printk("lguest: map_vm_area failed: %i\n", err);
goto free_vma;
}
+
+ /* Now the switcher is mapped at the right address, we can't fail!
+ * Copy in the compiled-in Switcher code (from switcher.S). */
memcpy(switcher_vma->addr, start_switcher_text,
end_switcher_text - start_switcher_text);
- /* Fix up IDT entries to point into copied text. */
+ /* Most of the switcher.S doesn't care that it's been moved; on Intel,
+ * jumps are relative, and it doesn't access any references to external
+ * code or data.
+ *
+ * The only exception is the interrupt handlers in switcher.S: their
+ * addresses are placed in a table (default_idt_entries), so we need to
+ * update the table with the new addresses. switcher_offset() is a
+ * convenience function which returns the distance between the builtin
+ * switcher code and the high-mapped copy we just made. */
for (i = 0; i < IDT_ENTRIES; i++)
default_idt_entries[i] += switcher_offset();
+ /*
+ * Set up the Switcher's per-cpu areas.
+ *
+ * Each CPU gets two pages of its own within the high-mapped region
+ * (aka. "struct lguest_pages"). Much of this can be initialized now,
+ * but some depends on what Guest we are running (which is set up in
+ * copy_in_guest_info()).
+ */
for_each_possible_cpu(i) {
+ /* lguest_pages() returns this CPU's two pages. */
struct lguest_pages *pages = lguest_pages(i);
+ /* This is a convenience pointer to make the code fit one
+ * statement to a line. */
struct lguest_ro_state *state = &pages->state;
- /* These fields are static: rest done in copy_in_guest_info */
+ /* The Global Descriptor Table: the Host has a different one
+ * for each CPU. We keep a descriptor for the GDT which says
+ * where it is and how big it is (the size is actually the last
+ * byte, not the size, hence the "-1"). */
state->host_gdt_desc.size = GDT_SIZE-1;
state->host_gdt_desc.address = (long)get_cpu_gdt_table(i);
+
+ /* All CPUs on the Host use the same Interrupt Descriptor
+ * Table, so we just use store_idt(), which gets this CPU's IDT
+ * descriptor. */
store_idt(&state->host_idt_desc);
+
+ /* The descriptors for the Guest's GDT and IDT can be filled
+ * out now, too. We copy the GDT & IDT into ->guest_gdt and
+ * ->guest_idt before actually running the Guest. */
state->guest_idt_desc.size = sizeof(state->guest_idt)-1;
state->guest_idt_desc.address = (long)&state->guest_idt;
state->guest_gdt_desc.size = sizeof(state->guest_gdt)-1;
state->guest_gdt_desc.address = (long)&state->guest_gdt;
+
+ /* We know where we want the stack to be when the Guest enters
+ * the switcher: in pages->regs. The stack grows upwards, so
+ * we start it at the end of that structure. */
state->guest_tss.esp0 = (long)(&pages->regs + 1);
+ /* And this is the GDT entry to use for the stack: we keep a
+ * couple of special LGUEST entries. */
state->guest_tss.ss0 = LGUEST_DS;
- /* No I/O for you! */
+
+ /* x86 can have a finegrained bitmap which indicates what I/O
+ * ports the process can use. We set it to the end of our
+ * structure, meaning "none". */
state->guest_tss.io_bitmap_base = sizeof(state->guest_tss);
+
+ /* Some GDT entries are the same across all Guests, so we can
+ * set them up now. */
setup_default_gdt_entries(state);
+ /* Most IDT entries are the same for all Guests, too.*/
setup_default_idt_entries(state, default_idt_entries);
- /* Setup LGUEST segments on all cpus */
+ /* The Host needs to be able to use the LGUEST segments on this
+ * CPU, too, so put them in the Host GDT. */
get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
}
- /* Initialize entry point into switcher. */
+ /* In the Switcher, we want the %cs segment register to use the
+ * LGUEST_CS GDT entry: we've put that in the Host and Guest GDTs, so
+ * it will be undisturbed when we switch. To change %cs and jump we
+ * need this structure to feed to Intel's "lcall" instruction. */
lguest_entry.offset = (long)switch_to_guest + switcher_offset();
lguest_entry.segment = LGUEST_CS;
printk(KERN_INFO "lguest: mapped switcher at %p\n",
switcher_vma->addr);
+ /* And we succeeded... */
return 0;
free_vma:
@@ -149,35 +232,58 @@
out:
return err;
}
+/*:*/
+/* Cleaning up the mapping when the module is unloaded is almost...
+ * too easy. */
static void unmap_switcher(void)
{
unsigned int i;
+ /* vunmap() undoes *both* map_vm_area() and __get_vm_area(). */
vunmap(switcher_vma->addr);
+ /* Now we just need to free the pages we copied the switcher into */
for (i = 0; i < TOTAL_SWITCHER_PAGES; i++)
__free_pages(switcher_page[i], 0);
}
-/* IN/OUT insns: enough to get us past boot-time probing. */
+/*H:130 Our Guest is usually so well behaved; it never tries to do things it
+ * isn't allowed to. Unfortunately, "struct paravirt_ops" isn't quite
+ * complete, because it doesn't contain replacements for the Intel I/O
+ * instructions. As a result, the Guest sometimes fumbles across one during
+ * the boot process as it probes for various things which are usually attached
+ * to a PC.
+ *
+ * When the Guest uses one of these instructions, we get trap #13 (General
+ * Protection Fault) and come here. We see if it's one of those troublesome
+ * instructions and skip over it. We return true if we did. */
static int emulate_insn(struct lguest *lg)
{
u8 insn;
unsigned int insnlen = 0, in = 0, shift = 0;
+ /* The eip contains the *virtual* address of the Guest's instruction:
+ * guest_pa just subtracts the Guest's page_offset. */
unsigned long physaddr = guest_pa(lg, lg->regs->eip);
- /* This only works for addresses in linear mapping... */
+ /* The guest_pa() function only works for Guest kernel addresses, but
+ * that's all we're trying to do anyway. */
if (lg->regs->eip < lg->page_offset)
return 0;
+
+ /* Decoding x86 instructions is icky. */
lgread(lg, &insn, physaddr, 1);
- /* Operand size prefix means it's actually for ax. */
+ /* 0x66 is an "operand prefix". It means it's using the upper 16 bits
+ of the eax register. */
if (insn == 0x66) {
shift = 16;
+ /* The instruction is 1 byte so far, read the next byte. */
insnlen = 1;
lgread(lg, &insn, physaddr + insnlen, 1);
}
+ /* We can ignore the lower bit for the moment and decode the 4 opcodes
+ * we need to emulate. */
switch (insn & 0xFE) {
case 0xE4: /* in <next byte>,%al */
insnlen += 2;
@@ -194,9 +300,13 @@
insnlen += 1;
break;
default:
+ /* OK, we don't know what this is, can't emulate. */
return 0;
}
+ /* If it was an "IN" instruction, they expect the result to be read
+ * into %eax, so we change %eax. We always return all-ones, which
+ * traditionally means "there's nothing there". */
if (in) {
/* Lower bit tells is whether it's a 16 or 32 bit access */
if (insn & 0x1)
@@ -204,9 +314,12 @@
else
lg->regs->eax |= (0xFFFF << shift);
}
+ /* Finally, we've "done" the instruction, so move past it. */
lg->regs->eip += insnlen;
+ /* Success! */
return 1;
}
+/*:*/
/*L:305
* Dealing With Guest Memory.
@@ -321,13 +434,24 @@
: "memory", "%edx", "%ecx", "%edi", "%esi");
}
+/*H:030 Let's jump straight to the the main loop which runs the Guest.
+ * Remember, this is called by the Launcher reading /dev/lguest, and we keep
+ * going around and around until something interesting happens. */
int run_guest(struct lguest *lg, unsigned long __user *user)
{
+ /* We stop running once the Guest is dead. */
while (!lg->dead) {
+ /* We need to initialize this, otherwise gcc complains. It's
+ * not (yet) clever enough to see that it's initialized when we
+ * need it. */
unsigned int cr2 = 0; /* Damn gcc */
- /* Hypercalls first: we might have been out to userspace */
+ /* First we run any hypercalls the Guest wants done: either in
+ * the hypercall ring in "struct lguest_data", or directly by
+ * using int 31 (LGUEST_TRAP_ENTRY). */
do_hypercalls(lg);
+ /* It's possible the Guest did a SEND_DMA hypercall to the
+ * Launcher, in which case we return from the read() now. */
if (lg->dma_is_pending) {
if (put_user(lg->pending_dma, user) ||
put_user(lg->pending_key, user+1))
@@ -335,6 +459,7 @@
return sizeof(unsigned long)*2;
}
+ /* Check for signals */
if (signal_pending(current))
return -ERESTARTSYS;
@@ -342,77 +467,154 @@
if (lg->break_out)
return -EAGAIN;
+ /* Check if there are any interrupts which can be delivered
+ * now: if so, this sets up the hander to be executed when we
+ * next run the Guest. */
maybe_do_interrupt(lg);
+ /* All long-lived kernel loops need to check with this horrible
+ * thing called the freezer. If the Host is trying to suspend,
+ * it stops us. */
try_to_freeze();
+ /* Just make absolutely sure the Guest is still alive. One of
+ * those hypercalls could have been fatal, for example. */
if (lg->dead)
break;
+ /* If the Guest asked to be stopped, we sleep. The Guest's
+ * clock timer or LHCALL_BREAK from the Waker will wake us. */
if (lg->halted) {
set_current_state(TASK_INTERRUPTIBLE);
schedule();
continue;
}
+ /* OK, now we're ready to jump into the Guest. First we put up
+ * the "Do Not Disturb" sign: */
local_irq_disable();
- /* Even if *we* don't want FPU trap, guest might... */
+ /* Remember the awfully-named TS bit? If the Guest has asked
+ * to set it we set it now, so we can trap and pass that trap
+ * to the Guest if it uses the FPU. */
if (lg->ts)
set_ts();
- /* Don't let Guest do SYSENTER: we can't handle it. */
+ /* SYSENTER is an optimized way of doing system calls. We
+ * can't allow it because it always jumps to privilege level 0.
+ * A normal Guest won't try it because we don't advertise it in
+ * CPUID, but a malicious Guest (or malicious Guest userspace
+ * program) could, so we tell the CPU to disable it before
+ * running the Guest. */
if (boot_cpu_has(X86_FEATURE_SEP))
wrmsr(MSR_IA32_SYSENTER_CS, 0, 0);
+ /* Now we actually run the Guest. It will pop back out when
+ * something interesting happens, and we can examine its
+ * registers to see what it was doing. */
run_guest_once(lg, lguest_pages(raw_smp_processor_id()));
- /* Save cr2 now if we page-faulted. */
+ /* The "regs" pointer contains two extra entries which are not
+ * really registers: a trap number which says what interrupt or
+ * trap made the switcher code come back, and an error code
+ * which some traps set. */
+
+ /* If the Guest page faulted, then the cr2 register will tell
+ * us the bad virtual address. We have to grab this now,
+ * because once we re-enable interrupts an interrupt could
+ * fault and thus overwrite cr2, or we could even move off to a
+ * different CPU. */
if (lg->regs->trapnum == 14)
cr2 = read_cr2();
+ /* Similarly, if we took a trap because the Guest used the FPU,
+ * we have to restore the FPU it expects to see. */
else if (lg->regs->trapnum == 7)
math_state_restore();
+ /* Restore SYSENTER if it's supposed to be on. */
if (boot_cpu_has(X86_FEATURE_SEP))
wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
+
+ /* Now we're ready to be interrupted or moved to other CPUs */
local_irq_enable();
+ /* OK, so what happened? */
switch (lg->regs->trapnum) {
case 13: /* We've intercepted a GPF. */
+ /* Check if this was one of those annoying IN or OUT
+ * instructions which we need to emulate. If so, we
+ * just go back into the Guest after we've done it. */
if (lg->regs->errcode == 0) {
if (emulate_insn(lg))
continue;
}
break;
case 14: /* We've intercepted a page fault. */
+ /* The Guest accessed a virtual address that wasn't
+ * mapped. This happens a lot: we don't actually set
+ * up most of the page tables for the Guest at all when
+ * we start: as it runs it asks for more and more, and
+ * we set them up as required. In this case, we don't
+ * even tell the Guest that the fault happened.
+ *
+ * The errcode tells whether this was a read or a
+ * write, and whether kernel or userspace code. */
if (demand_page(lg, cr2, lg->regs->errcode))
continue;
- /* If lguest_data is NULL, this won't hurt. */
+ /* OK, it's really not there (or not OK): the Guest
+ * needs to know. We write out the cr2 value so it
+ * knows where the fault occurred.
+ *
+ * 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))
kill_guest(lg, "Writing cr2");
break;
case 7: /* We've intercepted a Device Not Available fault. */
- /* If they don't want to know, just absorb it. */
+ /* If the Guest doesn't want to know, we already
+ * restored the Floating Point Unit, so we just
+ * continue without telling it. */
if (!lg->ts)
continue;
break;
- case 32 ... 255: /* Real interrupt, fall thru */
+ case 32 ... 255:
+ /* These values mean a real interrupt occurred, in
+ * which case the Host handler has already been run.
+ * We just do a friendly check if another process
+ * should now be run, then fall through to loop
+ * around: */
cond_resched();
case LGUEST_TRAP_ENTRY: /* Handled at top of loop */
continue;
}
+ /* If we get here, it's a trap the Guest wants to know
+ * about. */
if (deliver_trap(lg, lg->regs->trapnum))
continue;
+ /* If the Guest doesn't have a handler (either it hasn't
+ * registered any yet, or it's one of the faults we don't let
+ * it handle), it dies with a cryptic error message. */
kill_guest(lg, "unhandled trap %li at %#lx (%#lx)",
lg->regs->trapnum, lg->regs->eip,
lg->regs->trapnum == 14 ? cr2 : lg->regs->errcode);
}
+ /* The Guest is dead => "No such file or directory" */
return -ENOENT;
}
+/* Now we can look at each of the routines this calls, in increasing order of
+ * complexity: do_hypercalls(), emulate_insn(), maybe_do_interrupt(),
+ * deliver_trap() and demand_page(). After all those, we'll be ready to
+ * examine the Switcher, and our philosophical understanding of the Host/Guest
+ * duality will be complete. :*/
+
int find_free_guest(void)
{
unsigned int i;
@@ -430,55 +632,96 @@
write_cr4(read_cr4() & ~X86_CR4_PGE);
}
+/*H:000
+ * Welcome to the Host!
+ *
+ * By this point your brain has been tickled by the Guest code and numbed by
+ * the Launcher code; prepare for it to be stretched by the Host code. This is
+ * the heart. Let's begin at the initialization routine for the Host's lg
+ * module.
+ */
static int __init init(void)
{
int err;
+ /* Lguest can't run under Xen, VMI or itself. It does Tricky Stuff. */
if (paravirt_enabled()) {
printk("lguest is afraid of %s\n", paravirt_ops.name);
return -EPERM;
}
+ /* First we put the Switcher up in very high virtual memory. */
err = map_switcher();
if (err)
return err;
+ /* Now we set up the pagetable implementation for the Guests. */
err = init_pagetables(switcher_page, SHARED_SWITCHER_PAGES);
if (err) {
unmap_switcher();
return err;
}
+
+ /* The I/O subsystem needs some things initialized. */
lguest_io_init();
+ /* /dev/lguest needs to be registered. */
err = lguest_device_init();
if (err) {
free_pagetables();
unmap_switcher();
return err;
}
+
+ /* Finally, we need to turn off "Page Global Enable". PGE is an
+ * optimization where page table entries are specially marked to show
+ * they never change. The Host kernel marks all the kernel pages this
+ * way because it's always present, even when userspace is running.
+ *
+ * Lguest breaks this: unbeknownst to the rest of the Host kernel, we
+ * switch to the Guest kernel. If you don't disable this on all CPUs,
+ * you'll get really weird bugs that you'll chase for two days.
+ *
+ * I used to turn PGE off every time we switched to the Guest and back
+ * on when we return, but that slowed the Switcher down noticibly. */
+
+ /* We don't need the complexity of CPUs coming and going while we're
+ * doing this. */
lock_cpu_hotplug();
if (cpu_has_pge) { /* We have a broader idea of "global". */
+ /* Remember that this was originally set (for cleanup). */
cpu_had_pge = 1;
+ /* adjust_pge is a helper function which sets or unsets the PGE
+ * bit on its CPU, depending on the argument (0 == unset). */
on_each_cpu(adjust_pge, (void *)0, 0, 1);
+ /* Turn off the feature in the global feature set. */
clear_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability);
}
unlock_cpu_hotplug();
+
+ /* All good! */
return 0;
}
+/* Cleaning up is just the same code, backwards. With a little French. */
static void __exit fini(void)
{
lguest_device_remove();
free_pagetables();
unmap_switcher();
+
+ /* If we had PGE before we started, turn it back on now. */
lock_cpu_hotplug();
if (cpu_had_pge) {
set_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability);
+ /* adjust_pge's argument "1" means set PGE. */
on_each_cpu(adjust_pge, (void *)1, 0, 1);
}
unlock_cpu_hotplug();
}
+/* The Host side of lguest can be a module. This is a nice way for people to
+ * play with it. */
module_init(init);
module_exit(fini);
MODULE_LICENSE("GPL");