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/*P:800 Interrupts (traps) are complicated enough to earn their own file.
* There are three classes of interrupts:
* 1) Real hardware interrupts which occur while we're running the Guest,
* 2) Interrupts for virtual devices attached to the Guest, and
* 3) Traps and faults from the Guest.
* Real hardware interrupts must be delivered to the Host, not the Guest.
* Virtual interrupts must be delivered to the Guest, but we make them look
* just like real hardware would deliver them. Traps from the Guest can be set
* up to go directly back into the Guest, but sometimes the Host wants to see
* them first, so we also have a way of "reflecting" them into the Guest as if
* they had been delivered to it directly. :*/
#include <linux/uaccess.h>
#include <linux/interrupt.h>
#include <linux/module.h>
#include "lg.h"
/* Allow Guests to use a non-128 (ie. non-Linux) syscall trap. */
static unsigned int syscall_vector = SYSCALL_VECTOR;
module_param(syscall_vector, uint, 0444);
/* The address of the interrupt handler is split into two bits: */
static unsigned long idt_address(u32 lo, u32 hi)
return (lo & 0x0000FFFF) | (hi & 0xFFFF0000);
/* The "type" of the interrupt handler is a 4 bit field: we only support a
* couple of types. */
static int idt_type(u32 lo, u32 hi)
return (hi >> 8) & 0xF;
/* An IDT entry can't be used unless the "present" bit is set. */
static int idt_present(u32 lo, u32 hi)
return (hi & 0x8000);
/* We need a helper to "push" a value onto the Guest's stack, since that's a
* big part of what delivering an interrupt does. */
static void push_guest_stack(struct lg_cpu *cpu, unsigned long *gstack, u32 val)
/* Stack grows upwards: move stack then write value. */
*gstack -= 4;
lgwrite(cpu, *gstack, u32, val);
/*H:210 The set_guest_interrupt() routine actually delivers the interrupt or
* trap. The mechanics of delivering traps and interrupts to the Guest are the
* same, except some traps have an "error code" which gets pushed onto the
* stack as well: the caller tells us if this is one.
* "lo" and "hi" are the two parts of the Interrupt Descriptor Table for this
* interrupt or trap. It's split into two parts for traditional reasons: gcc
* on i386 used to be frightened by 64 bit numbers.
* We set up the stack just like the CPU does for a real interrupt, so it's
* identical for the Guest (and the standard "iret" instruction will undo
* it). */
static void set_guest_interrupt(struct lg_cpu *cpu, u32 lo, u32 hi, int has_err)
unsigned long gstack, origstack;
u32 eflags, ss, irq_enable;
unsigned long virtstack;
/* There are two cases for interrupts: one where the Guest is already
* in the kernel, and a more complex one where the Guest is in
* userspace. We check the privilege level to find out. */
if ((cpu->regs->ss&0x3) != GUEST_PL) {
/* The Guest told us their kernel stack with the SET_STACK
* hypercall: both the virtual address and the segment */
virtstack = cpu->esp1;
ss = cpu->ss1;
origstack = gstack = guest_pa(cpu, virtstack);
/* We push the old stack segment and pointer onto the new
* stack: when the Guest does an "iret" back from the interrupt
* handler the CPU will notice they're dropping privilege
* levels and expect these here. */
push_guest_stack(cpu, &gstack, cpu->regs->ss);
push_guest_stack(cpu, &gstack, cpu->regs->esp);
} else {
/* We're staying on the same Guest (kernel) stack. */
virtstack = cpu->regs->esp;
ss = cpu->regs->ss;
origstack = gstack = guest_pa(cpu, virtstack);
/* Remember that we never let the Guest actually disable interrupts, so
* the "Interrupt Flag" bit is always set. We copy that bit from the
* Guest's "irq_enabled" field into the eflags word: we saw the Guest
* copy it back in "lguest_iret". */
eflags = cpu->regs->eflags;
if (get_user(irq_enable, &cpu->lg->lguest_data->irq_enabled) == 0
&& !(irq_enable & X86_EFLAGS_IF))
eflags &= ~X86_EFLAGS_IF;
/* An interrupt is expected to push three things on the stack: the old
* "eflags" word, the old code segment, and the old instruction
* pointer. */
push_guest_stack(cpu, &gstack, eflags);
push_guest_stack(cpu, &gstack, cpu->regs->cs);
push_guest_stack(cpu, &gstack, cpu->regs->eip);
/* For the six traps which supply an error code, we push that, too. */
if (has_err)
push_guest_stack(cpu, &gstack, cpu->regs->errcode);
/* Now we've pushed all the old state, we change the stack, the code
* segment and the address to execute. */
cpu->regs->ss = ss;
cpu->regs->esp = virtstack + (gstack - origstack);
cpu->regs->cs = (__KERNEL_CS|GUEST_PL);
cpu->regs->eip = idt_address(lo, hi);
/* There are two kinds of interrupt handlers: 0xE is an "interrupt
* gate" which expects interrupts to be disabled on entry. */
if (idt_type(lo, hi) == 0xE)
if (put_user(0, &cpu->lg->lguest_data->irq_enabled))
kill_guest(cpu, "Disabling interrupts");
* Virtual Interrupts.
* maybe_do_interrupt() gets called before every entry to the Guest, to see if
* we should divert the Guest to running an interrupt handler. */
void maybe_do_interrupt(struct lg_cpu *cpu)
unsigned int irq;
struct desc_struct *idt;
/* If the Guest hasn't even initialized yet, we can do nothing. */
if (!cpu->lg->lguest_data)
/* Take our "irqs_pending" array and remove any interrupts the Guest
* wants blocked: the result ends up in "blk". */
if (copy_from_user(&blk, cpu->lg->lguest_data->blocked_interrupts,
bitmap_andnot(blk, cpu->irqs_pending, blk, LGUEST_IRQS);
/* Find the first interrupt. */
irq = find_first_bit(blk, LGUEST_IRQS);
/* None? Nothing to do */
if (irq >= LGUEST_IRQS)
/* They may be in the middle of an iret, where they asked us never to
* deliver interrupts. */
if (cpu->regs->eip >= cpu->lg->noirq_start &&
(cpu->regs->eip < cpu->lg->noirq_end))
/* If they're halted, interrupts restart them. */
if (cpu->halted) {
/* Re-enable interrupts. */
if (put_user(X86_EFLAGS_IF, &cpu->lg->lguest_data->irq_enabled))
kill_guest(cpu, "Re-enabling interrupts");
cpu->halted = 0;
} else {
/* Otherwise we check if they have interrupts disabled. */
u32 irq_enabled;
if (get_user(irq_enabled, &cpu->lg->lguest_data->irq_enabled))
irq_enabled = 0;
if (!irq_enabled)
/* Look at the IDT entry the Guest gave us for this interrupt. The
* first 32 (FIRST_EXTERNAL_VECTOR) entries are for traps, so we skip
* over them. */
idt = &cpu->arch.idt[FIRST_EXTERNAL_VECTOR+irq];
/* If they don't have a handler (yet?), we just ignore it */
if (idt_present(idt->a, idt->b)) {
/* OK, mark it no longer pending and deliver it. */
clear_bit(irq, cpu->irqs_pending);
/* set_guest_interrupt() takes the interrupt descriptor and a
* flag to say whether this interrupt pushes an error code onto
* the stack as well: virtual interrupts never do. */
set_guest_interrupt(cpu, idt->a, idt->b, 0);
/* Every time we deliver an interrupt, we update the timestamp in the
* Guest's lguest_data struct. It would be better for the Guest if we
* did this more often, but it can actually be quite slow: doing it
* here is a compromise which means at least it gets updated every
* timer interrupt. */
/* Linux uses trap 128 for system calls. Plan9 uses 64, and Ron Minnich sent
* me a patch, so we support that too. It'd be a big step for lguest if half
* the Plan 9 user base were to start using it.
* Actually now I think of it, it's possible that Ron *is* half the Plan 9
* userbase. Oh well. */
static bool could_be_syscall(unsigned int num)
/* Normal Linux SYSCALL_VECTOR or reserved vector? */
return num == SYSCALL_VECTOR || num == syscall_vector;
/* The syscall vector it wants must be unused by Host. */
bool check_syscall_vector(struct lguest *lg)
u32 vector;
if (get_user(vector, &lg->lguest_data->syscall_vec))
return false;
return could_be_syscall(vector);
int init_interrupts(void)
/* If they want some strange system call vector, reserve it now */
if (syscall_vector != SYSCALL_VECTOR
&& test_and_set_bit(syscall_vector, used_vectors)) {
printk("lg: couldn't reserve syscall %u\n", syscall_vector);
return -EBUSY;
return 0;
void free_interrupts(void)
if (syscall_vector != SYSCALL_VECTOR)
clear_bit(syscall_vector, used_vectors);
/*H:220 Now we've got the routines to deliver interrupts, delivering traps like
* page fault is easy. The only trick is that Intel decided that some traps
* should have error codes: */
static int has_err(unsigned int trap)
return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17);
/* deliver_trap() returns true if it could deliver the trap. */
int deliver_trap(struct lg_cpu *cpu, unsigned int num)
/* Trap numbers are always 8 bit, but we set an impossible trap number
* for traps inside the Switcher, so check that here. */
if (num >= ARRAY_SIZE(cpu->arch.idt))
return 0;
/* Early on the Guest hasn't set the IDT entries (or maybe it put a
* bogus one in): if we fail here, the Guest will be killed. */
if (!idt_present(cpu->arch.idt[num].a, cpu->arch.idt[num].b))
return 0;
set_guest_interrupt(cpu, cpu->arch.idt[num].a,
cpu->arch.idt[num].b, has_err(num));
return 1;
/*H:250 Here's the hard part: returning to the Host every time a trap happens
* and then calling deliver_trap() and re-entering the Guest is slow.
* Particularly because Guest userspace system calls are traps (usually trap
* 128).
* So we'd like to set up the IDT to tell the CPU to deliver traps directly
* into the Guest. This is possible, but the complexities cause the size of
* this file to double! However, 150 lines of code is worth writing for taking
* system calls down from 1750ns to 270ns. Plus, if lguest didn't do it, all
* the other hypervisors would beat it up at lunchtime.
* This routine indicates if a particular trap number could be delivered
* directly. */
static int direct_trap(unsigned int num)
/* Hardware interrupts don't go to the Guest at all (except system
* call). */
if (num >= FIRST_EXTERNAL_VECTOR && !could_be_syscall(num))
return 0;
/* The Host needs to see page faults (for shadow paging and to save the
* fault address), general protection faults (in/out emulation) and
* device not available (TS handling), and of course, the hypercall
* trap. */
return num != 14 && num != 13 && num != 7 && num != LGUEST_TRAP_ENTRY;
/*M:005 The Guest has the ability to turn its interrupt gates into trap gates,
* if it is careful. The Host will let trap gates can go directly to the
* Guest, but the Guest needs the interrupts atomically disabled for an
* interrupt gate. It can do this by pointing the trap gate at instructions
* within noirq_start and noirq_end, where it can safely disable interrupts. */
/*M:006 The Guests do not use the sysenter (fast system call) instruction,
* because it's hardcoded to enter privilege level 0 and so can't go direct.
* It's about twice as fast as the older "int 0x80" system call, so it might
* still be worthwhile to handle it in the Switcher and lcall down to the
* Guest. The sysenter semantics are hairy tho: search for that keyword in
* entry.S :*/
/*H:260 When we make traps go directly into the Guest, we need to make sure
* the kernel stack is valid (ie. mapped in the page tables). Otherwise, the
* CPU trying to deliver the trap will fault while trying to push the interrupt
* words on the stack: this is called a double fault, and it forces us to kill
* the Guest.
* Which is deeply unfair, because (literally!) it wasn't the Guests' fault. */
void pin_stack_pages(struct lg_cpu *cpu)
unsigned int i;
/* Depending on the CONFIG_4KSTACKS option, the Guest can have one or
* two pages of stack space. */
for (i = 0; i < cpu->lg->stack_pages; i++)
/* The stack grows *upwards*, so the address we're given is the
* start of the page after the kernel stack. Subtract one to
* get back onto the first stack page, and keep subtracting to
* get to the rest of the stack pages. */
pin_page(cpu, cpu->esp1 - 1 - i * PAGE_SIZE);
/* Direct traps also mean that we need to know whenever the Guest wants to use
* a different kernel stack, so we can change the IDT entries to use that
* stack. The IDT entries expect a virtual address, so unlike most addresses
* the Guest gives us, the "esp" (stack pointer) value here is virtual, not
* physical.
* In Linux each process has its own kernel stack, so this happens a lot: we
* change stacks on each context switch. */
void guest_set_stack(struct lg_cpu *cpu, u32 seg, u32 esp, unsigned int pages)
/* You are not allowed have a stack segment with privilege level 0: bad
* Guest! */
if ((seg & 0x3) != GUEST_PL)
kill_guest(cpu, "bad stack segment %i", seg);
/* We only expect one or two stack pages. */
if (pages > 2)
kill_guest(cpu, "bad stack pages %u", pages);
/* Save where the stack is, and how many pages */
cpu->ss1 = seg;
cpu->esp1 = esp;
cpu->lg->stack_pages = pages;
/* Make sure the new stack pages are mapped */
/* All this reference to mapping stacks leads us neatly into the other complex
* part of the Host: page table handling. */
/*H:235 This is the routine which actually checks the Guest's IDT entry and
* transfers it into the entry in "struct lguest": */
static void set_trap(struct lg_cpu *cpu, struct desc_struct *trap,
unsigned int num, u32 lo, u32 hi)
u8 type = idt_type(lo, hi);
/* We zero-out a not-present entry */
if (!idt_present(lo, hi)) {
trap->a = trap->b = 0;
/* We only support interrupt and trap gates. */
if (type != 0xE && type != 0xF)
kill_guest(cpu, "bad IDT type %i", type);
/* We only copy the handler address, present bit, privilege level and
* type. The privilege level controls where the trap can be triggered
* manually with an "int" instruction. This is usually GUEST_PL,
* except for system calls which userspace can use. */
trap->a = ((__KERNEL_CS|GUEST_PL)<<16) | (lo&0x0000FFFF);
trap->b = (hi&0xFFFFEF00);
/*H:230 While we're here, dealing with delivering traps and interrupts to the
* Guest, we might as well complete the picture: how the Guest tells us where
* it wants them to go. This would be simple, except making traps fast
* requires some tricks.
* We saw the Guest setting Interrupt Descriptor Table (IDT) entries with the
* LHCALL_LOAD_IDT_ENTRY hypercall before: that comes here. */
void load_guest_idt_entry(struct lg_cpu *cpu, unsigned int num, u32 lo, u32 hi)
/* Guest never handles: NMI, doublefault, spurious interrupt or
* hypercall. We ignore when it tries to set them. */
if (num == 2 || num == 8 || num == 15 || num == LGUEST_TRAP_ENTRY)
/* Mark the IDT as changed: next time the Guest runs we'll know we have
* to copy this again. */
cpu->changed |= CHANGED_IDT;
/* Check that the Guest doesn't try to step outside the bounds. */
if (num >= ARRAY_SIZE(cpu->arch.idt))
kill_guest(cpu, "Setting idt entry %u", num);
set_trap(cpu, &cpu->arch.idt[num], num, lo, hi);
/* The default entry for each interrupt points into the Switcher routines which
* simply return to the Host. The run_guest() loop will then call
* deliver_trap() to bounce it back into the Guest. */
static void default_idt_entry(struct desc_struct *idt,
int trap,
const unsigned long handler,
const struct desc_struct *base)
/* A present interrupt gate. */
u32 flags = 0x8e00;
/* Set the privilege level on the entry for the hypercall: this allows
* the Guest to use the "int" instruction to trigger it. */
if (trap == LGUEST_TRAP_ENTRY)
flags |= (GUEST_PL << 13);
else if (base)
/* Copy priv. level from what Guest asked for. This allows
* debug (int 3) traps from Guest userspace, for example. */
flags |= (base->b & 0x6000);
/* Now pack it into the IDT entry in its weird format. */
idt->a = (LGUEST_CS<<16) | (handler&0x0000FFFF);
idt->b = (handler&0xFFFF0000) | flags;
/* When the Guest first starts, we put default entries into the IDT. */
void setup_default_idt_entries(struct lguest_ro_state *state,
const unsigned long *def)
unsigned int i;
for (i = 0; i < ARRAY_SIZE(state->guest_idt); i++)
default_idt_entry(&state->guest_idt[i], i, def[i], NULL);
/*H:240 We don't use the IDT entries in the "struct lguest" directly, instead
* we copy them into the IDT which we've set up for Guests on this CPU, just
* before we run the Guest. This routine does that copy. */
void copy_traps(const struct lg_cpu *cpu, struct desc_struct *idt,
const unsigned long *def)
unsigned int i;
/* We can simply copy the direct traps, otherwise we use the default
* ones in the Switcher: they will return to the Host. */
for (i = 0; i < ARRAY_SIZE(cpu->arch.idt); i++) {
const struct desc_struct *gidt = &cpu->arch.idt[i];
/* If no Guest can ever override this trap, leave it alone. */
if (!direct_trap(i))
/* Only trap gates (type 15) can go direct to the Guest.
* Interrupt gates (type 14) disable interrupts as they are
* entered, which we never let the Guest do. Not present
* entries (type 0x0) also can't go direct, of course.
* If it can't go direct, we still need to copy the priv. level:
* they might want to give userspace access to a software
* interrupt. */
if (idt_type(gidt->a, gidt->b) == 0xF)
idt[i] = *gidt;
default_idt_entry(&idt[i], i, def[i], gidt);
* The Guest Clock.
* There are two sources of virtual interrupts. We saw one in lguest_user.c:
* the Launcher sending interrupts for virtual devices. The other is the Guest
* timer interrupt.
* The Guest uses the LHCALL_SET_CLOCKEVENT hypercall to tell us how long to
* the next timer interrupt (in nanoseconds). We use the high-resolution timer
* infrastructure to set a callback at that time.
* 0 means "turn off the clock". */
void guest_set_clockevent(struct lg_cpu *cpu, unsigned long delta)
ktime_t expires;
if (unlikely(delta == 0)) {
/* Clock event device is shutting down. */
/* We use wallclock time here, so the Guest might not be running for
* all the time between now and the timer interrupt it asked for. This
* is almost always the right thing to do. */
expires = ktime_add_ns(ktime_get_real(), delta);
hrtimer_start(&cpu->hrt, expires, HRTIMER_MODE_ABS);
/* This is the function called when the Guest's timer expires. */
static enum hrtimer_restart clockdev_fn(struct hrtimer *timer)
struct lg_cpu *cpu = container_of(timer, struct lg_cpu, hrt);
/* Remember the first interrupt is the timer interrupt. */
set_bit(0, cpu->irqs_pending);
/* If the Guest is actually stopped, we need to wake it up. */
if (cpu->halted)
/* This sets up the timer for this Guest. */
void init_clockdev(struct lg_cpu *cpu)
hrtimer_init(&cpu->hrt, CLOCK_REALTIME, HRTIMER_MODE_ABS);
cpu->hrt.function = clockdev_fn;