447 lines
16 KiB
C
447 lines
16 KiB
C
/*P:800 Interrupts (traps) are complicated enough to earn their own file.
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* There are three classes of interrupts:
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*
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* 1) Real hardware interrupts which occur while we're running the Guest,
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* 2) Interrupts for virtual devices attached to the Guest, and
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* 3) Traps and faults from the Guest.
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*
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* Real hardware interrupts must be delivered to the Host, not the Guest.
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* Virtual interrupts must be delivered to the Guest, but we make them look
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* just like real hardware would deliver them. Traps from the Guest can be set
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* up to go directly back into the Guest, but sometimes the Host wants to see
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* them first, so we also have a way of "reflecting" them into the Guest as if
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* they had been delivered to it directly. :*/
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#include <linux/uaccess.h>
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#include "lg.h"
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/* The address of the interrupt handler is split into two bits: */
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static unsigned long idt_address(u32 lo, u32 hi)
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{
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return (lo & 0x0000FFFF) | (hi & 0xFFFF0000);
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}
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/* The "type" of the interrupt handler is a 4 bit field: we only support a
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* couple of types. */
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static int idt_type(u32 lo, u32 hi)
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{
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return (hi >> 8) & 0xF;
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}
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/* An IDT entry can't be used unless the "present" bit is set. */
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static int idt_present(u32 lo, u32 hi)
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{
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return (hi & 0x8000);
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}
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/* We need a helper to "push" a value onto the Guest's stack, since that's a
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* big part of what delivering an interrupt does. */
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static void push_guest_stack(struct lguest *lg, unsigned long *gstack, u32 val)
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{
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/* Stack grows upwards: move stack then write value. */
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*gstack -= 4;
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lgwrite_u32(lg, *gstack, val);
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}
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/*H:210 The set_guest_interrupt() routine actually delivers the interrupt or
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* trap. The mechanics of delivering traps and interrupts to the Guest are the
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* same, except some traps have an "error code" which gets pushed onto the
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* stack as well: the caller tells us if this is one.
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*
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* "lo" and "hi" are the two parts of the Interrupt Descriptor Table for this
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* interrupt or trap. It's split into two parts for traditional reasons: gcc
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* on i386 used to be frightened by 64 bit numbers.
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*
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* We set up the stack just like the CPU does for a real interrupt, so it's
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* identical for the Guest (and the standard "iret" instruction will undo
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* it). */
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static void set_guest_interrupt(struct lguest *lg, u32 lo, u32 hi, int has_err)
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{
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unsigned long gstack;
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u32 eflags, ss, irq_enable;
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/* There are two cases for interrupts: one where the Guest is already
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* in the kernel, and a more complex one where the Guest is in
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* userspace. We check the privilege level to find out. */
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if ((lg->regs->ss&0x3) != GUEST_PL) {
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/* The Guest told us their kernel stack with the SET_STACK
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* hypercall: both the virtual address and the segment */
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gstack = guest_pa(lg, lg->esp1);
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ss = lg->ss1;
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/* We push the old stack segment and pointer onto the new
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* stack: when the Guest does an "iret" back from the interrupt
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* handler the CPU will notice they're dropping privilege
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* levels and expect these here. */
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push_guest_stack(lg, &gstack, lg->regs->ss);
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push_guest_stack(lg, &gstack, lg->regs->esp);
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} else {
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/* We're staying on the same Guest (kernel) stack. */
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gstack = guest_pa(lg, lg->regs->esp);
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ss = lg->regs->ss;
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}
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/* Remember that we never let the Guest actually disable interrupts, so
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* the "Interrupt Flag" bit is always set. We copy that bit from the
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* Guest's "irq_enabled" field into the eflags word: the Guest copies
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* it back in "lguest_iret". */
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eflags = lg->regs->eflags;
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if (get_user(irq_enable, &lg->lguest_data->irq_enabled) == 0
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&& !(irq_enable & X86_EFLAGS_IF))
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eflags &= ~X86_EFLAGS_IF;
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/* An interrupt is expected to push three things on the stack: the old
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* "eflags" word, the old code segment, and the old instruction
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* pointer. */
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push_guest_stack(lg, &gstack, eflags);
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push_guest_stack(lg, &gstack, lg->regs->cs);
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push_guest_stack(lg, &gstack, lg->regs->eip);
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/* For the six traps which supply an error code, we push that, too. */
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if (has_err)
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push_guest_stack(lg, &gstack, lg->regs->errcode);
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/* Now we've pushed all the old state, we change the stack, the code
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* segment and the address to execute. */
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lg->regs->ss = ss;
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lg->regs->esp = gstack + lg->page_offset;
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lg->regs->cs = (__KERNEL_CS|GUEST_PL);
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lg->regs->eip = idt_address(lo, hi);
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/* There are two kinds of interrupt handlers: 0xE is an "interrupt
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* gate" which expects interrupts to be disabled on entry. */
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if (idt_type(lo, hi) == 0xE)
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if (put_user(0, &lg->lguest_data->irq_enabled))
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kill_guest(lg, "Disabling interrupts");
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}
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/*H:200
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* Virtual Interrupts.
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*
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* maybe_do_interrupt() gets called before every entry to the Guest, to see if
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* we should divert the Guest to running an interrupt handler. */
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void maybe_do_interrupt(struct lguest *lg)
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{
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unsigned int irq;
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DECLARE_BITMAP(blk, LGUEST_IRQS);
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struct desc_struct *idt;
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/* If the Guest hasn't even initialized yet, we can do nothing. */
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if (!lg->lguest_data)
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return;
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/* Take our "irqs_pending" array and remove any interrupts the Guest
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* wants blocked: the result ends up in "blk". */
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if (copy_from_user(&blk, lg->lguest_data->blocked_interrupts,
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sizeof(blk)))
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return;
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bitmap_andnot(blk, lg->irqs_pending, blk, LGUEST_IRQS);
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/* Find the first interrupt. */
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irq = find_first_bit(blk, LGUEST_IRQS);
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/* None? Nothing to do */
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if (irq >= LGUEST_IRQS)
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return;
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/* They may be in the middle of an iret, where they asked us never to
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* deliver interrupts. */
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if (lg->regs->eip >= lg->noirq_start && lg->regs->eip < lg->noirq_end)
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return;
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/* If they're halted, interrupts restart them. */
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if (lg->halted) {
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/* Re-enable interrupts. */
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if (put_user(X86_EFLAGS_IF, &lg->lguest_data->irq_enabled))
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kill_guest(lg, "Re-enabling interrupts");
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lg->halted = 0;
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} else {
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/* Otherwise we check if they have interrupts disabled. */
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u32 irq_enabled;
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if (get_user(irq_enabled, &lg->lguest_data->irq_enabled))
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irq_enabled = 0;
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if (!irq_enabled)
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return;
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}
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/* Look at the IDT entry the Guest gave us for this interrupt. The
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* first 32 (FIRST_EXTERNAL_VECTOR) entries are for traps, so we skip
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* over them. */
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idt = &lg->idt[FIRST_EXTERNAL_VECTOR+irq];
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/* If they don't have a handler (yet?), we just ignore it */
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if (idt_present(idt->a, idt->b)) {
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/* OK, mark it no longer pending and deliver it. */
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clear_bit(irq, lg->irqs_pending);
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/* set_guest_interrupt() takes the interrupt descriptor and a
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* flag to say whether this interrupt pushes an error code onto
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* the stack as well: virtual interrupts never do. */
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set_guest_interrupt(lg, idt->a, idt->b, 0);
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}
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/* Every time we deliver an interrupt, we update the timestamp in the
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* Guest's lguest_data struct. It would be better for the Guest if we
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* did this more often, but it can actually be quite slow: doing it
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* here is a compromise which means at least it gets updated every
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* timer interrupt. */
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write_timestamp(lg);
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}
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/*H:220 Now we've got the routines to deliver interrupts, delivering traps
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* like page fault is easy. The only trick is that Intel decided that some
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* traps should have error codes: */
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static int has_err(unsigned int trap)
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{
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return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17);
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}
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/* deliver_trap() returns true if it could deliver the trap. */
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int deliver_trap(struct lguest *lg, unsigned int num)
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{
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/* Trap numbers are always 8 bit, but we set an impossible trap number
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* for traps inside the Switcher, so check that here. */
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if (num >= ARRAY_SIZE(lg->idt))
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return 0;
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/* Early on the Guest hasn't set the IDT entries (or maybe it put a
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* bogus one in): if we fail here, the Guest will be killed. */
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if (!idt_present(lg->idt[num].a, lg->idt[num].b))
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return 0;
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set_guest_interrupt(lg, lg->idt[num].a, lg->idt[num].b, has_err(num));
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return 1;
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}
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/*H:250 Here's the hard part: returning to the Host every time a trap happens
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* and then calling deliver_trap() and re-entering the Guest is slow.
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* Particularly because Guest userspace system calls are traps (trap 128).
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*
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* So we'd like to set up the IDT to tell the CPU to deliver traps directly
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* into the Guest. This is possible, but the complexities cause the size of
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* this file to double! However, 150 lines of code is worth writing for taking
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* system calls down from 1750ns to 270ns. Plus, if lguest didn't do it, all
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* the other hypervisors would tease it.
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*
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* This routine determines if a trap can be delivered directly. */
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static int direct_trap(const struct lguest *lg,
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const struct desc_struct *trap,
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unsigned int num)
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{
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/* Hardware interrupts don't go to the Guest at all (except system
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* call). */
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if (num >= FIRST_EXTERNAL_VECTOR && num != SYSCALL_VECTOR)
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return 0;
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/* The Host needs to see page faults (for shadow paging and to save the
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* fault address), general protection faults (in/out emulation) and
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* device not available (TS handling), and of course, the hypercall
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* trap. */
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if (num == 14 || num == 13 || num == 7 || num == LGUEST_TRAP_ENTRY)
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return 0;
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/* Only trap gates (type 15) can go direct to the Guest. Interrupt
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* gates (type 14) disable interrupts as they are entered, which we
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* never let the Guest do. Not present entries (type 0x0) also can't
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* go direct, of course 8) */
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return idt_type(trap->a, trap->b) == 0xF;
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}
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/*:*/
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/*M:005 The Guest has the ability to turn its interrupt gates into trap gates,
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* if it is careful. The Host will let trap gates can go directly to the
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* Guest, but the Guest needs the interrupts atomically disabled for an
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* interrupt gate. It can do this by pointing the trap gate at instructions
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* within noirq_start and noirq_end, where it can safely disable interrupts. */
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/*M:006 The Guests do not use the sysenter (fast system call) instruction,
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* because it's hardcoded to enter privilege level 0 and so can't go direct.
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* It's about twice as fast as the older "int 0x80" system call, so it might
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* still be worthwhile to handle it in the Switcher and lcall down to the
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* Guest. The sysenter semantics are hairy tho: search for that keyword in
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* entry.S :*/
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/*H:260 When we make traps go directly into the Guest, we need to make sure
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* the kernel stack is valid (ie. mapped in the page tables). Otherwise, the
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* CPU trying to deliver the trap will fault while trying to push the interrupt
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* words on the stack: this is called a double fault, and it forces us to kill
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* the Guest.
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*
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* Which is deeply unfair, because (literally!) it wasn't the Guests' fault. */
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void pin_stack_pages(struct lguest *lg)
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{
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unsigned int i;
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/* Depending on the CONFIG_4KSTACKS option, the Guest can have one or
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* two pages of stack space. */
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for (i = 0; i < lg->stack_pages; i++)
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/* The stack grows *upwards*, so the address we're given is the
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* start of the page after the kernel stack. Subtract one to
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* get back onto the first stack page, and keep subtracting to
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* get to the rest of the stack pages. */
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pin_page(lg, lg->esp1 - 1 - i * PAGE_SIZE);
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}
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/* Direct traps also mean that we need to know whenever the Guest wants to use
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* a different kernel stack, so we can change the IDT entries to use that
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* stack. The IDT entries expect a virtual address, so unlike most addresses
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* the Guest gives us, the "esp" (stack pointer) value here is virtual, not
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* physical.
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*
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* In Linux each process has its own kernel stack, so this happens a lot: we
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* change stacks on each context switch. */
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void guest_set_stack(struct lguest *lg, u32 seg, u32 esp, unsigned int pages)
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{
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/* You are not allowd have a stack segment with privilege level 0: bad
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* Guest! */
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if ((seg & 0x3) != GUEST_PL)
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kill_guest(lg, "bad stack segment %i", seg);
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/* We only expect one or two stack pages. */
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if (pages > 2)
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kill_guest(lg, "bad stack pages %u", pages);
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/* Save where the stack is, and how many pages */
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lg->ss1 = seg;
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lg->esp1 = esp;
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lg->stack_pages = pages;
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/* Make sure the new stack pages are mapped */
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pin_stack_pages(lg);
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}
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/* All this reference to mapping stacks leads us neatly into the other complex
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* part of the Host: page table handling. */
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/*H:235 This is the routine which actually checks the Guest's IDT entry and
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* transfers it into our entry in "struct lguest": */
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static void set_trap(struct lguest *lg, struct desc_struct *trap,
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unsigned int num, u32 lo, u32 hi)
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{
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u8 type = idt_type(lo, hi);
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/* We zero-out a not-present entry */
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if (!idt_present(lo, hi)) {
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trap->a = trap->b = 0;
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return;
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}
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/* We only support interrupt and trap gates. */
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if (type != 0xE && type != 0xF)
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kill_guest(lg, "bad IDT type %i", type);
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/* We only copy the handler address, present bit, privilege level and
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* type. The privilege level controls where the trap can be triggered
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* manually with an "int" instruction. This is usually GUEST_PL,
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* except for system calls which userspace can use. */
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trap->a = ((__KERNEL_CS|GUEST_PL)<<16) | (lo&0x0000FFFF);
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trap->b = (hi&0xFFFFEF00);
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}
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/*H:230 While we're here, dealing with delivering traps and interrupts to the
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* Guest, we might as well complete the picture: how the Guest tells us where
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* it wants them to go. This would be simple, except making traps fast
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* requires some tricks.
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*
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* We saw the Guest setting Interrupt Descriptor Table (IDT) entries with the
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* LHCALL_LOAD_IDT_ENTRY hypercall before: that comes here. */
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void load_guest_idt_entry(struct lguest *lg, unsigned int num, u32 lo, u32 hi)
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{
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/* Guest never handles: NMI, doublefault, spurious interrupt or
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* hypercall. We ignore when it tries to set them. */
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if (num == 2 || num == 8 || num == 15 || num == LGUEST_TRAP_ENTRY)
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return;
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/* Mark the IDT as changed: next time the Guest runs we'll know we have
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* to copy this again. */
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lg->changed |= CHANGED_IDT;
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/* The IDT which we keep in "struct lguest" only contains 32 entries
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* for the traps and LGUEST_IRQS (32) entries for interrupts. We
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* ignore attempts to set handlers for higher interrupt numbers, except
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* for the system call "interrupt" at 128: we have a special IDT entry
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* for that. */
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if (num < ARRAY_SIZE(lg->idt))
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set_trap(lg, &lg->idt[num], num, lo, hi);
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else if (num == SYSCALL_VECTOR)
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set_trap(lg, &lg->syscall_idt, num, lo, hi);
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}
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/* The default entry for each interrupt points into the Switcher routines which
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* simply return to the Host. The run_guest() loop will then call
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* deliver_trap() to bounce it back into the Guest. */
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static void default_idt_entry(struct desc_struct *idt,
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int trap,
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const unsigned long handler)
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{
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/* A present interrupt gate. */
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u32 flags = 0x8e00;
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/* Set the privilege level on the entry for the hypercall: this allows
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* the Guest to use the "int" instruction to trigger it. */
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if (trap == LGUEST_TRAP_ENTRY)
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flags |= (GUEST_PL << 13);
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/* Now pack it into the IDT entry in its weird format. */
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idt->a = (LGUEST_CS<<16) | (handler&0x0000FFFF);
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idt->b = (handler&0xFFFF0000) | flags;
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}
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/* When the Guest first starts, we put default entries into the IDT. */
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void setup_default_idt_entries(struct lguest_ro_state *state,
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const unsigned long *def)
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{
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unsigned int i;
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for (i = 0; i < ARRAY_SIZE(state->guest_idt); i++)
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default_idt_entry(&state->guest_idt[i], i, def[i]);
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}
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/*H:240 We don't use the IDT entries in the "struct lguest" directly, instead
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* we copy them into the IDT which we've set up for Guests on this CPU, just
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* before we run the Guest. This routine does that copy. */
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void copy_traps(const struct lguest *lg, struct desc_struct *idt,
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const unsigned long *def)
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{
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unsigned int i;
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/* We can simply copy the direct traps, otherwise we use the default
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* ones in the Switcher: they will return to the Host. */
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for (i = 0; i < FIRST_EXTERNAL_VECTOR; i++) {
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if (direct_trap(lg, &lg->idt[i], i))
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idt[i] = lg->idt[i];
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else
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default_idt_entry(&idt[i], i, def[i]);
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}
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/* Don't forget the system call trap! The IDT entries for other
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* interupts never change, so no need to copy them. */
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i = SYSCALL_VECTOR;
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if (direct_trap(lg, &lg->syscall_idt, i))
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idt[i] = lg->syscall_idt;
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else
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default_idt_entry(&idt[i], i, def[i]);
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}
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void guest_set_clockevent(struct lguest *lg, unsigned long delta)
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{
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ktime_t expires;
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if (unlikely(delta == 0)) {
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/* Clock event device is shutting down. */
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hrtimer_cancel(&lg->hrt);
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return;
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}
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expires = ktime_add_ns(ktime_get_real(), delta);
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hrtimer_start(&lg->hrt, expires, HRTIMER_MODE_ABS);
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}
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static enum hrtimer_restart clockdev_fn(struct hrtimer *timer)
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{
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struct lguest *lg = container_of(timer, struct lguest, hrt);
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set_bit(0, lg->irqs_pending);
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if (lg->halted)
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wake_up_process(lg->tsk);
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return HRTIMER_NORESTART;
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}
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void init_clockdev(struct lguest *lg)
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{
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hrtimer_init(&lg->hrt, CLOCK_REALTIME, HRTIMER_MODE_ABS);
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lg->hrt.function = clockdev_fn;
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}
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