blob: af65b2da3ba0dabd19d0e14fcfbb52577ff4c96c [file] [log] [blame]
* A hypervisor allows multiple Operating Systems to run on a single machine.
* To quote David Wheeler: "Any problem in computer science can be solved with
* another layer of indirection."
* We keep things simple in two ways. First, we start with a normal Linux
* kernel and insert a module (lg.ko) which allows us to run other Linux
* kernels the same way we'd run processes. We call the first kernel the Host,
* and the others the Guests. The program which sets up and configures Guests
* (such as the example in Documentation/lguest/lguest.c) is called the
* Launcher.
* Secondly, we only run specially modified Guests, not normal kernels: setting
* CONFIG_LGUEST_GUEST to "y" compiles this file into the kernel so it knows
* how to be a Guest at boot time. This means that you can use the same kernel
* you boot normally (ie. as a Host) as a Guest.
* These Guests know that they cannot do privileged operations, such as disable
* interrupts, and that they have to ask the Host to do such things explicitly.
* This file consists of all the replacements for such low-level native
* hardware operations: these special Guest versions call the Host.
* So how does the kernel know it's a Guest? We'll see that later, but let's
* just say that we end up here where we replace the native functions various
* "paravirt" structures with our Guest versions, then boot like normal. :*/
* Copyright (C) 2006, Rusty Russell <> IBM Corporation.
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* NON INFRINGEMENT. See the GNU General Public License for more
* details.
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
#include <linux/kernel.h>
#include <linux/start_kernel.h>
#include <linux/string.h>
#include <linux/console.h>
#include <linux/screen_info.h>
#include <linux/irq.h>
#include <linux/interrupt.h>
#include <linux/clocksource.h>
#include <linux/clockchips.h>
#include <linux/lguest.h>
#include <linux/lguest_launcher.h>
#include <linux/virtio_console.h>
#include <linux/pm.h>
#include <asm/lguest.h>
#include <asm/paravirt.h>
#include <asm/param.h>
#include <asm/page.h>
#include <asm/pgtable.h>
#include <asm/desc.h>
#include <asm/setup.h>
#include <asm/e820.h>
#include <asm/mce.h>
#include <asm/io.h>
#include <asm/i387.h>
#include <asm/reboot.h> /* for struct machine_ops */
/*G:010 Welcome to the Guest!
* The Guest in our tale is a simple creature: identical to the Host but
* behaving in simplified but equivalent ways. In particular, the Guest is the
* same kernel as the Host (or at least, built from the same source code). :*/
struct lguest_data lguest_data = {
.hcall_status = { [0 ... LHCALL_RING_SIZE-1] = 0xFF },
.noirq_start = (u32)lguest_noirq_start,
.noirq_end = (u32)lguest_noirq_end,
.kernel_address = PAGE_OFFSET,
.blocked_interrupts = { 1 }, /* Block timer interrupts */
.syscall_vec = SYSCALL_VECTOR,
/*G:037 async_hcall() is pretty simple: I'm quite proud of it really. We have a
* ring buffer of stored hypercalls which the Host will run though next time we
* do a normal hypercall. Each entry in the ring has 4 slots for the hypercall
* arguments, and a "hcall_status" word which is 0 if the call is ready to go,
* and 255 once the Host has finished with it.
* If we come around to a slot which hasn't been finished, then the table is
* full and we just make the hypercall directly. This has the nice side
* effect of causing the Host to run all the stored calls in the ring buffer
* which empties it for next time! */
static void async_hcall(unsigned long call, unsigned long arg1,
unsigned long arg2, unsigned long arg3)
/* Note: This code assumes we're uniprocessor. */
static unsigned int next_call;
unsigned long flags;
/* Disable interrupts if not already disabled: we don't want an
* interrupt handler making a hypercall while we're already doing
* one! */
if (lguest_data.hcall_status[next_call] != 0xFF) {
/* Table full, so do normal hcall which will flush table. */
hcall(call, arg1, arg2, arg3);
} else {
lguest_data.hcalls[next_call].arg0 = call;
lguest_data.hcalls[next_call].arg1 = arg1;
lguest_data.hcalls[next_call].arg2 = arg2;
lguest_data.hcalls[next_call].arg3 = arg3;
/* Arguments must all be written before we mark it to go */
lguest_data.hcall_status[next_call] = 0;
if (++next_call == LHCALL_RING_SIZE)
next_call = 0;
/*G:035 Notice the lazy_hcall() above, rather than hcall(). This is our first
* real optimization trick!
* When lazy_mode is set, it means we're allowed to defer all hypercalls and do
* them as a batch when lazy_mode is eventually turned off. Because hypercalls
* are reasonably expensive, batching them up makes sense. For example, a
* large munmap might update dozens of page table entries: that code calls
* paravirt_enter_lazy_mmu(), does the dozen updates, then calls
* lguest_leave_lazy_mode().
* So, when we're in lazy mode, we call async_hcall() to store the call for
* future processing: */
static void lazy_hcall(unsigned long call,
unsigned long arg1,
unsigned long arg2,
unsigned long arg3)
if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE)
hcall(call, arg1, arg2, arg3);
async_hcall(call, arg1, arg2, arg3);
/* When lazy mode is turned off reset the per-cpu lazy mode variable and then
* issue the do-nothing hypercall to flush any stored calls. */
static void lguest_leave_lazy_mode(void)
hcall(LHCALL_FLUSH_ASYNC, 0, 0, 0);
* After that diversion we return to our first native-instruction
* replacements: four functions for interrupt control.
* The simplest way of implementing these would be to have "turn interrupts
* off" and "turn interrupts on" hypercalls. Unfortunately, this is too slow:
* these are by far the most commonly called functions of those we override.
* So instead we keep an "irq_enabled" field inside our "struct lguest_data",
* which the Guest can update with a single instruction. The Host knows to
* check there before it tries to deliver an interrupt.
/* save_flags() is expected to return the processor state (ie. "flags"). The
* flags word contains all kind of stuff, but in practice Linux only cares
* about the interrupt flag. Our "save_flags()" just returns that. */
static unsigned long save_fl(void)
return lguest_data.irq_enabled;
/* restore_flags() just sets the flags back to the value given. */
static void restore_fl(unsigned long flags)
lguest_data.irq_enabled = flags;
/* Interrupts go off... */
static void irq_disable(void)
lguest_data.irq_enabled = 0;
/* Interrupts go on... */
static void irq_enable(void)
lguest_data.irq_enabled = X86_EFLAGS_IF;
/*M:003 Note that we don't check for outstanding interrupts when we re-enable
* them (or when we unmask an interrupt). This seems to work for the moment,
* since interrupts are rare and we'll just get the interrupt on the next timer
* tick, but now we can run with CONFIG_NO_HZ, we should revisit this. One way
* would be to put the "irq_enabled" field in a page by itself, and have the
* Host write-protect it when an interrupt comes in when irqs are disabled.
* There will then be a page fault as soon as interrupts are re-enabled.
* A better method is to implement soft interrupt disable generally for x86:
* instead of disabling interrupts, we set a flag. If an interrupt does come
* in, we then disable them for real. This is uncommon, so we could simply use
* a hypercall for interrupt control and not worry about efficiency. :*/
* The Interrupt Descriptor Table (IDT).
* The IDT tells the processor what to do when an interrupt comes in. Each
* entry in the table is a 64-bit descriptor: this holds the privilege level,
* address of the handler, and... well, who cares? The Guest just asks the
* Host to make the change anyway, because the Host controls the real IDT.
static void lguest_write_idt_entry(gate_desc *dt,
int entrynum, const gate_desc *g)
/* The gate_desc structure is 8 bytes long: we hand it to the Host in
* two 32-bit chunks. The whole 32-bit kernel used to hand descriptors
* around like this; typesafety wasn't a big concern in Linux's early
* years. */
u32 *desc = (u32 *)g;
/* Keep the local copy up to date. */
native_write_idt_entry(dt, entrynum, g);
/* Tell Host about this new entry. */
hcall(LHCALL_LOAD_IDT_ENTRY, entrynum, desc[0], desc[1]);
/* Changing to a different IDT is very rare: we keep the IDT up-to-date every
* time it is written, so we can simply loop through all entries and tell the
* Host about them. */
static void lguest_load_idt(const struct desc_ptr *desc)
unsigned int i;
struct desc_struct *idt = (void *)desc->address;
for (i = 0; i < (desc->size+1)/8; i++)
hcall(LHCALL_LOAD_IDT_ENTRY, i, idt[i].a, idt[i].b);
* The Global Descriptor Table.
* The Intel architecture defines another table, called the Global Descriptor
* Table (GDT). You tell the CPU where it is (and its size) using the "lgdt"
* instruction, and then several other instructions refer to entries in the
* table. There are three entries which the Switcher needs, so the Host simply
* controls the entire thing and the Guest asks it to make changes using the
* LOAD_GDT hypercall.
* This is the opposite of the IDT code where we have a LOAD_IDT_ENTRY
* hypercall and use that repeatedly to load a new IDT. I don't think it
* really matters, but wouldn't it be nice if they were the same? Wouldn't
* it be even better if you were the one to send the patch to fix it?
static void lguest_load_gdt(const struct desc_ptr *desc)
BUG_ON((desc->size+1)/8 != GDT_ENTRIES);
hcall(LHCALL_LOAD_GDT, __pa(desc->address), GDT_ENTRIES, 0);
/* For a single GDT entry which changes, we do the lazy thing: alter our GDT,
* then tell the Host to reload the entire thing. This operation is so rare
* that this naive implementation is reasonable. */
static void lguest_write_gdt_entry(struct desc_struct *dt, int entrynum,
const void *desc, int type)
native_write_gdt_entry(dt, entrynum, desc, type);
hcall(LHCALL_LOAD_GDT, __pa(dt), GDT_ENTRIES, 0);
/* OK, I lied. There are three "thread local storage" GDT entries which change
* on every context switch (these three entries are how glibc implements
* __thread variables). So we have a hypercall specifically for this case. */
static void lguest_load_tls(struct thread_struct *t, unsigned int cpu)
/* There's one problem which normal hardware doesn't have: the Host
* can't handle us removing entries we're currently using. So we clear
* the GS register here: if it's needed it'll be reloaded anyway. */
loadsegment(gs, 0);
lazy_hcall(LHCALL_LOAD_TLS, __pa(&t->tls_array), cpu, 0);
/*G:038 That's enough excitement for now, back to ploughing through each of
* the different pv_ops structures (we're about 1/3 of the way through).
* This is the Local Descriptor Table, another weird Intel thingy. Linux only
* uses this for some strange applications like Wine. We don't do anything
* here, so they'll get an informative and friendly Segmentation Fault. */
static void lguest_set_ldt(const void *addr, unsigned entries)
/* This loads a GDT entry into the "Task Register": that entry points to a
* structure called the Task State Segment. Some comments scattered though the
* kernel code indicate that this used for task switching in ages past, along
* with blood sacrifice and astrology.
* Now there's nothing interesting in here that we don't get told elsewhere.
* But the native version uses the "ltr" instruction, which makes the Host
* complain to the Guest about a Segmentation Fault and it'll oops. So we
* override the native version with a do-nothing version. */
static void lguest_load_tr_desc(void)
/* The "cpuid" instruction is a way of querying both the CPU identity
* (manufacturer, model, etc) and its features. It was introduced before the
* Pentium in 1993 and keeps getting extended by both Intel, AMD and others.
* As you might imagine, after a decade and a half this treatment, it is now a
* giant ball of hair. Its entry in the current Intel manual runs to 28 pages.
* This instruction even it has its own Wikipedia entry. The Wikipedia entry
* has been translated into 4 languages. I am not making this up!
* We could get funky here and identify ourselves as "GenuineLguest", but
* instead we just use the real "cpuid" instruction. Then I pretty much turned
* off feature bits until the Guest booted. (Don't say that: you'll damage
* lguest sales!) Shut up, inner voice! (Hey, just pointing out that this is
* hardly future proof.) Noone's listening! They don't like you anyway,
* parenthetic weirdo!
* Replacing the cpuid so we can turn features off is great for the kernel, but
* anyone (including userspace) can just use the raw "cpuid" instruction and
* the Host won't even notice since it isn't privileged. So we try not to get
* too worked up about it. */
static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
unsigned int *cx, unsigned int *dx)
int function = *ax;
native_cpuid(ax, bx, cx, dx);
switch (function) {
case 1: /* Basic feature request. */
/* We only allow kernel to see SSE3, CMPXCHG16B and SSSE3 */
*cx &= 0x00002201;
*dx &= 0x07808111;
/* The Host can do a nice optimization if it knows that the
* kernel mappings (addresses above 0xC0000000 or whatever
* PAGE_OFFSET is set to) haven't changed. But Linux calls
* flush_tlb_user() for both user and kernel mappings unless
* the Page Global Enable (PGE) feature bit is set. */
*dx |= 0x00002000;
case 0x80000000:
/* Futureproof this a little: if they ask how much extended
* processor information there is, limit it to known fields. */
if (*ax > 0x80000008)
*ax = 0x80000008;
/* Intel has four control registers, imaginatively named cr0, cr2, cr3 and cr4.
* I assume there's a cr1, but it hasn't bothered us yet, so we'll not bother
* it. The Host needs to know when the Guest wants to change them, so we have
* a whole series of functions like read_cr0() and write_cr0().
* We start with cr0. cr0 allows you to turn on and off all kinds of basic
* features, but Linux only really cares about one: the horrifically-named Task
* Switched (TS) bit at bit 3 (ie. 8)
* What does the TS bit do? Well, it causes the CPU to trap (interrupt 7) if
* the floating point unit is used. Which allows us to restore FPU state
* lazily after a task switch, and Linux uses that gratefully, but wouldn't a
* name like "FPUTRAP bit" be a little less cryptic?
* We store cr0 (and cr3) locally, because the Host never changes it. The
* Guest sometimes wants to read it and we'd prefer not to bother the Host
* unnecessarily. */
static unsigned long current_cr0, current_cr3;
static void lguest_write_cr0(unsigned long val)
lazy_hcall(LHCALL_TS, val & X86_CR0_TS, 0, 0);
current_cr0 = val;
static unsigned long lguest_read_cr0(void)
return current_cr0;
/* Intel provided a special instruction to clear the TS bit for people too cool
* to use write_cr0() to do it. This "clts" instruction is faster, because all
* the vowels have been optimized out. */
static void lguest_clts(void)
lazy_hcall(LHCALL_TS, 0, 0, 0);
current_cr0 &= ~X86_CR0_TS;
/* cr2 is the virtual address of the last page fault, which the Guest only ever
* reads. The Host kindly writes this into our "struct lguest_data", so we
* just read it out of there. */
static unsigned long lguest_read_cr2(void)
return lguest_data.cr2;
/* cr3 is the current toplevel pagetable page: the principle is the same as
* cr0. Keep a local copy, and tell the Host when it changes. */
static void lguest_write_cr3(unsigned long cr3)
lazy_hcall(LHCALL_NEW_PGTABLE, cr3, 0, 0);
current_cr3 = cr3;
static unsigned long lguest_read_cr3(void)
return current_cr3;
/* cr4 is used to enable and disable PGE, but we don't care. */
static unsigned long lguest_read_cr4(void)
return 0;
static void lguest_write_cr4(unsigned long val)
* Page Table Handling.
* Now would be a good time to take a rest and grab a coffee or similarly
* relaxing stimulant. The easy parts are behind us, and the trek gradually
* winds uphill from here.
* Quick refresher: memory is divided into "pages" of 4096 bytes each. The CPU
* maps virtual addresses to physical addresses using "page tables". We could
* use one huge index of 1 million entries: each address is 4 bytes, so that's
* 1024 pages just to hold the page tables. But since most virtual addresses
* are unused, we use a two level index which saves space. The cr3 register
* contains the physical address of the top level "page directory" page, which
* contains physical addresses of up to 1024 second-level pages. Each of these
* second level pages contains up to 1024 physical addresses of actual pages,
* or Page Table Entries (PTEs).
* Here's a diagram, where arrows indicate physical addresses:
* cr3 ---> +---------+
* | --------->+---------+
* | | | PADDR1 |
* Top-level | | PADDR2 |
* (PMD) page | | |
* | | Lower-level |
* | | (PTE) page |
* | | | |
* .... ....
* So to convert a virtual address to a physical address, we look up the top
* level, which points us to the second level, which gives us the physical
* address of that page. If the top level entry was not present, or the second
* level entry was not present, then the virtual address is invalid (we
* say "the page was not mapped").
* Put another way, a 32-bit virtual address is divided up like so:
* 1 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
* |<---- 10 bits ---->|<---- 10 bits ---->|<------ 12 bits ------>|
* Index into top Index into second Offset within page
* page directory page pagetable page
* The kernel spends a lot of time changing both the top-level page directory
* and lower-level pagetable pages. The Guest doesn't know physical addresses,
* so while it maintains these page tables exactly like normal, it also needs
* to keep the Host informed whenever it makes a change: the Host will create
* the real page tables based on the Guests'.
/* The Guest calls this to set a second-level entry (pte), ie. to map a page
* into a process' address space. We set the entry then tell the Host the
* toplevel and address this corresponds to. The Guest uses one pagetable per
* process, so we need to tell the Host which one we're changing (mm->pgd). */
static void lguest_set_pte_at(struct mm_struct *mm, unsigned long addr,
pte_t *ptep, pte_t pteval)
*ptep = pteval;
lazy_hcall(LHCALL_SET_PTE, __pa(mm->pgd), addr, pteval.pte_low);
/* The Guest calls this to set a top-level entry. Again, we set the entry then
* tell the Host which top-level page we changed, and the index of the entry we
* changed. */
static void lguest_set_pmd(pmd_t *pmdp, pmd_t pmdval)
*pmdp = pmdval;
lazy_hcall(LHCALL_SET_PMD, __pa(pmdp)&PAGE_MASK,
(__pa(pmdp)&(PAGE_SIZE-1))/4, 0);
/* There are a couple of legacy places where the kernel sets a PTE, but we
* don't know the top level any more. This is useless for us, since we don't
* know which pagetable is changing or what address, so we just tell the Host
* to forget all of them. Fortunately, this is very rare.
* ... except in early boot when the kernel sets up the initial pagetables,
* which makes booting astonishingly slow. So we don't even tell the Host
* anything changed until we've done the first page table switch. */
static void lguest_set_pte(pte_t *ptep, pte_t pteval)
*ptep = pteval;
/* Don't bother with hypercall before initial setup. */
if (current_cr3)
lazy_hcall(LHCALL_FLUSH_TLB, 1, 0, 0);
/* Unfortunately for Lguest, the pv_mmu_ops for page tables were based on
* native page table operations. On native hardware you can set a new page
* table entry whenever you want, but if you want to remove one you have to do
* a TLB flush (a TLB is a little cache of page table entries kept by the CPU).
* So the lguest_set_pte_at() and lguest_set_pmd() functions above are only
* called when a valid entry is written, not when it's removed (ie. marked not
* present). Instead, this is where we come when the Guest wants to remove a
* page table entry: we tell the Host to set that entry to 0 (ie. the present
* bit is zero). */
static void lguest_flush_tlb_single(unsigned long addr)
/* Simply set it to zero: if it was not, it will fault back in. */
lazy_hcall(LHCALL_SET_PTE, current_cr3, addr, 0);
/* This is what happens after the Guest has removed a large number of entries.
* This tells the Host that any of the page table entries for userspace might
* have changed, ie. virtual addresses below PAGE_OFFSET. */
static void lguest_flush_tlb_user(void)
lazy_hcall(LHCALL_FLUSH_TLB, 0, 0, 0);
/* This is called when the kernel page tables have changed. That's not very
* common (unless the Guest is using highmem, which makes the Guest extremely
* slow), so it's worth separating this from the user flushing above. */
static void lguest_flush_tlb_kernel(void)
lazy_hcall(LHCALL_FLUSH_TLB, 1, 0, 0);
* The Unadvanced Programmable Interrupt Controller.
* This is an attempt to implement the simplest possible interrupt controller.
* I spent some time looking though routines like set_irq_chip_and_handler,
* set_irq_chip_and_handler_name, set_irq_chip_data and set_phasers_to_stun and
* I *think* this is as simple as it gets.
* We can tell the Host what interrupts we want blocked ready for using the
* lguest_data.interrupts bitmap, so disabling (aka "masking") them is as
* simple as setting a bit. We don't actually "ack" interrupts as such, we
* just mask and unmask them. I wonder if we should be cleverer?
static void disable_lguest_irq(unsigned int irq)
set_bit(irq, lguest_data.blocked_interrupts);
static void enable_lguest_irq(unsigned int irq)
clear_bit(irq, lguest_data.blocked_interrupts);
/* This structure describes the lguest IRQ controller. */
static struct irq_chip lguest_irq_controller = {
.name = "lguest",
.mask = disable_lguest_irq,
.mask_ack = disable_lguest_irq,
.unmask = enable_lguest_irq,
/* This sets up the Interrupt Descriptor Table (IDT) entry for each hardware
* interrupt (except 128, which is used for system calls), and then tells the
* Linux infrastructure that each interrupt is controlled by our level-based
* lguest interrupt controller. */
static void __init lguest_init_IRQ(void)
unsigned int i;
for (i = 0; i < LGUEST_IRQS; i++) {
int vector = FIRST_EXTERNAL_VECTOR + i;
if (vector != SYSCALL_VECTOR) {
set_intr_gate(vector, interrupt[i]);
set_irq_chip_and_handler(i, &lguest_irq_controller,
/* This call is required to set up for 4k stacks, where we have
* separate stacks for hard and soft interrupts. */
* Time.
* It would be far better for everyone if the Guest had its own clock, but
* until then the Host gives us the time on every interrupt.
static unsigned long lguest_get_wallclock(void)
return lguest_data.time.tv_sec;
/* The TSC is an Intel thing called the Time Stamp Counter. The Host tells us
* what speed it runs at, or 0 if it's unusable as a reliable clock source.
* This matches what we want here: if we return 0 from this function, the x86
* TSC clock will give up and not register itself. */
static unsigned long lguest_cpu_khz(void)
return lguest_data.tsc_khz;
/* If we can't use the TSC, the kernel falls back to our lower-priority
* "lguest_clock", where we read the time value given to us by the Host. */
static cycle_t lguest_clock_read(void)
unsigned long sec, nsec;
/* Since the time is in two parts (seconds and nanoseconds), we risk
* reading it just as it's changing from 99 & 0.999999999 to 100 and 0,
* and getting 99 and 0. As Linux tends to come apart under the stress
* of time travel, we must be careful: */
do {
/* First we read the seconds part. */
sec = lguest_data.time.tv_sec;
/* This read memory barrier tells the compiler and the CPU that
* this can't be reordered: we have to complete the above
* before going on. */
/* Now we read the nanoseconds part. */
nsec = lguest_data.time.tv_nsec;
/* Make sure we've done that. */
/* Now if the seconds part has changed, try again. */
} while (unlikely(lguest_data.time.tv_sec != sec));
/* Our lguest clock is in real nanoseconds. */
return sec*1000000000ULL + nsec;
/* This is the fallback clocksource: lower priority than the TSC clocksource. */
static struct clocksource lguest_clock = {
.name = "lguest",
.rating = 200,
.read = lguest_clock_read,
.mult = 1 << 22,
.shift = 22,
/* We also need a "struct clock_event_device": Linux asks us to set it to go
* off some time in the future. Actually, James Morris figured all this out, I
* just applied the patch. */
static int lguest_clockevent_set_next_event(unsigned long delta,
struct clock_event_device *evt)
/* FIXME: I don't think this can ever happen, but James tells me he had
* to put this code in. Maybe we should remove it now. Anyone? */
if (delta < LG_CLOCK_MIN_DELTA) {
if (printk_ratelimit())
printk(KERN_DEBUG "%s: small delta %lu ns\n",
__func__, delta);
return -ETIME;
/* Please wake us this far in the future. */
hcall(LHCALL_SET_CLOCKEVENT, delta, 0, 0);
return 0;
static void lguest_clockevent_set_mode(enum clock_event_mode mode,
struct clock_event_device *evt)
switch (mode) {
/* A 0 argument shuts the clock down. */
hcall(LHCALL_SET_CLOCKEVENT, 0, 0, 0);
/* This is what we expect. */
/* This describes our primitive timer chip. */
static struct clock_event_device lguest_clockevent = {
.name = "lguest",
.set_next_event = lguest_clockevent_set_next_event,
.set_mode = lguest_clockevent_set_mode,
.rating = INT_MAX,
.mult = 1,
.shift = 0,
.min_delta_ns = LG_CLOCK_MIN_DELTA,
.max_delta_ns = LG_CLOCK_MAX_DELTA,
/* This is the Guest timer interrupt handler (hardware interrupt 0). We just
* call the clockevent infrastructure and it does whatever needs doing. */
static void lguest_time_irq(unsigned int irq, struct irq_desc *desc)
unsigned long flags;
/* Don't interrupt us while this is running. */
/* At some point in the boot process, we get asked to set up our timing
* infrastructure. The kernel doesn't expect timer interrupts before this, but
* we cleverly initialized the "blocked_interrupts" field of "struct
* lguest_data" so that timer interrupts were blocked until now. */
static void lguest_time_init(void)
/* Set up the timer interrupt (0) to go to our simple timer routine */
set_irq_handler(0, lguest_time_irq);
/* We can't set cpumask in the initializer: damn C limitations! Set it
* here and register our timer device. */
lguest_clockevent.cpumask = cpumask_of_cpu(0);
/* Finally, we unblock the timer interrupt. */
* Miscellaneous bits and pieces.
* Here is an oddball collection of functions which the Guest needs for things
* to work. They're pretty simple.
/* The Guest needs to tell the Host what stack it expects traps to use. For
* native hardware, this is part of the Task State Segment mentioned above in
* lguest_load_tr_desc(), but to help hypervisors there's this special call.
* We tell the Host the segment we want to use (__KERNEL_DS is the kernel data
* segment), the privilege level (we're privilege level 1, the Host is 0 and
* will not tolerate us trying to use that), the stack pointer, and the number
* of pages in the stack. */
static void lguest_load_sp0(struct tss_struct *tss,
struct thread_struct *thread)
lazy_hcall(LHCALL_SET_STACK, __KERNEL_DS|0x1, thread->sp0,
/* Let's just say, I wouldn't do debugging under a Guest. */
static void lguest_set_debugreg(int regno, unsigned long value)
/* FIXME: Implement */
/* There are times when the kernel wants to make sure that no memory writes are
* caught in the cache (that they've all reached real hardware devices). This
* doesn't matter for the Guest which has virtual hardware.
* On the Pentium 4 and above, cpuid() indicates that the Cache Line Flush
* (clflush) instruction is available and the kernel uses that. Otherwise, it
* uses the older "Write Back and Invalidate Cache" (wbinvd) instruction.
* Unlike clflush, wbinvd can only be run at privilege level 0. So we can
* ignore clflush, but replace wbinvd.
static void lguest_wbinvd(void)
/* If the Guest expects to have an Advanced Programmable Interrupt Controller,
* we play dumb by ignoring writes and returning 0 for reads. So it's no
* longer Programmable nor Controlling anything, and I don't think 8 lines of
* code qualifies for Advanced. It will also never interrupt anything. It
* does, however, allow us to get through the Linux boot code. */
static void lguest_apic_write(unsigned long reg, u32 v)
static u32 lguest_apic_read(unsigned long reg)
return 0;
/* STOP! Until an interrupt comes in. */
static void lguest_safe_halt(void)
hcall(LHCALL_HALT, 0, 0, 0);
/* The SHUTDOWN hypercall takes a string to describe what's happening, and
* an argument which says whether this to restart (reboot) the Guest or not.
* Note that the Host always prefers that the Guest speak in physical addresses
* rather than virtual addresses, so we use __pa() here. */
static void lguest_power_off(void)
* Panicing.
* Don't. But if you did, this is what happens.
static int lguest_panic(struct notifier_block *nb, unsigned long l, void *p)
/* The hcall won't return, but to keep gcc happy, we're "done". */
static struct notifier_block paniced = {
.notifier_call = lguest_panic
/* Setting up memory is fairly easy. */
static __init char *lguest_memory_setup(void)
/* We do this here and not earlier because lockcheck used to barf if we
* did it before start_kernel(). I think we fixed that, so it'd be
* nice to move it back to lguest_init. Patch welcome... */
atomic_notifier_chain_register(&panic_notifier_list, &paniced);
/* The Linux bootloader header contains an "e820" memory map: the
* Launcher populated the first entry with our memory limit. */
/* This string is for the boot messages. */
return "LGUEST";
/* We will eventually use the virtio console device to produce console output,
* but before that is set up we use LHCALL_NOTIFY on normal memory to produce
* console output. */
static __init int early_put_chars(u32 vtermno, const char *buf, int count)
char scratch[17];
unsigned int len = count;
/* We use a nul-terminated string, so we have to make a copy. Icky,
* huh? */
if (len > sizeof(scratch) - 1)
len = sizeof(scratch) - 1;
scratch[len] = '\0';
memcpy(scratch, buf, len);
hcall(LHCALL_NOTIFY, __pa(scratch), 0, 0);
/* This routine returns the number of bytes actually written. */
return len;
/* Rebooting also tells the Host we're finished, but the RESTART flag tells the
* Launcher to reboot us. */
static void lguest_restart(char *reason)
* Patching (Powerfully Placating Performance Pedants)
* We have already seen that pv_ops structures let us replace simple native
* instructions with calls to the appropriate back end all throughout the
* kernel. This allows the same kernel to run as a Guest and as a native
* kernel, but it's slow because of all the indirect branches.
* Remember that David Wheeler quote about "Any problem in computer science can
* be solved with another layer of indirection"? The rest of that quote is
* "... But that usually will create another problem." This is the first of
* those problems.
* Our current solution is to allow the paravirt back end to optionally patch
* over the indirect calls to replace them with something more efficient. We
* patch the four most commonly called functions: disable interrupts, enable
* interrupts, restore interrupts and save interrupts. We usually have 6 or 10
* bytes to patch into: the Guest versions of these operations are small enough
* that we can fit comfortably.
* First we need assembly templates of each of the patchable Guest operations,
* and these are in lguest_asm.S. */
/*G:060 We construct a table from the assembler templates: */
static const struct lguest_insns
const char *start, *end;
} lguest_insns[] = {
[PARAVIRT_PATCH(pv_irq_ops.irq_disable)] = { lgstart_cli, lgend_cli },
[PARAVIRT_PATCH(pv_irq_ops.irq_enable)] = { lgstart_sti, lgend_sti },
[PARAVIRT_PATCH(pv_irq_ops.restore_fl)] = { lgstart_popf, lgend_popf },
[PARAVIRT_PATCH(pv_irq_ops.save_fl)] = { lgstart_pushf, lgend_pushf },
/* Now our patch routine is fairly simple (based on the native one in
* paravirt.c). If we have a replacement, we copy it in and return how much of
* the available space we used. */
static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf,
unsigned long addr, unsigned len)
unsigned int insn_len;
/* Don't do anything special if we don't have a replacement */
if (type >= ARRAY_SIZE(lguest_insns) || !lguest_insns[type].start)
return paravirt_patch_default(type, clobber, ibuf, addr, len);
insn_len = lguest_insns[type].end - lguest_insns[type].start;
/* Similarly if we can't fit replacement (shouldn't happen, but let's
* be thorough). */
if (len < insn_len)
return paravirt_patch_default(type, clobber, ibuf, addr, len);
/* Copy in our instructions. */
memcpy(ibuf, lguest_insns[type].start, insn_len);
return insn_len;
/*G:030 Once we get to lguest_init(), we know we're a Guest. The various
* pv_ops structures in the kernel provide points for (almost) every routine we
* have to override to avoid privileged instructions. */
__init void lguest_init(void)
/* We're under lguest, paravirt is enabled, and we're running at
* privilege level 1, not 0 as normal. */ = "lguest";
pv_info.paravirt_enabled = 1;
pv_info.kernel_rpl = 1;
/* We set up all the lguest overrides for sensitive operations. These
* are detailed with the operations themselves. */
/* interrupt-related operations */
pv_irq_ops.init_IRQ = lguest_init_IRQ;
pv_irq_ops.save_fl = save_fl;
pv_irq_ops.restore_fl = restore_fl;
pv_irq_ops.irq_disable = irq_disable;
pv_irq_ops.irq_enable = irq_enable;
pv_irq_ops.safe_halt = lguest_safe_halt;
/* init-time operations */
pv_init_ops.memory_setup = lguest_memory_setup;
pv_init_ops.patch = lguest_patch;
/* Intercepts of various cpu instructions */
pv_cpu_ops.load_gdt = lguest_load_gdt;
pv_cpu_ops.cpuid = lguest_cpuid;
pv_cpu_ops.load_idt = lguest_load_idt;
pv_cpu_ops.iret = lguest_iret;
pv_cpu_ops.load_sp0 = lguest_load_sp0;
pv_cpu_ops.load_tr_desc = lguest_load_tr_desc;
pv_cpu_ops.set_ldt = lguest_set_ldt;
pv_cpu_ops.load_tls = lguest_load_tls;
pv_cpu_ops.set_debugreg = lguest_set_debugreg;
pv_cpu_ops.clts = lguest_clts;
pv_cpu_ops.read_cr0 = lguest_read_cr0;
pv_cpu_ops.write_cr0 = lguest_write_cr0;
pv_cpu_ops.read_cr4 = lguest_read_cr4;
pv_cpu_ops.write_cr4 = lguest_write_cr4;
pv_cpu_ops.write_gdt_entry = lguest_write_gdt_entry;
pv_cpu_ops.write_idt_entry = lguest_write_idt_entry;
pv_cpu_ops.wbinvd = lguest_wbinvd;
pv_cpu_ops.lazy_mode.enter = paravirt_enter_lazy_cpu;
pv_cpu_ops.lazy_mode.leave = lguest_leave_lazy_mode;
/* pagetable management */
pv_mmu_ops.write_cr3 = lguest_write_cr3;
pv_mmu_ops.flush_tlb_user = lguest_flush_tlb_user;
pv_mmu_ops.flush_tlb_single = lguest_flush_tlb_single;
pv_mmu_ops.flush_tlb_kernel = lguest_flush_tlb_kernel;
pv_mmu_ops.set_pte = lguest_set_pte;
pv_mmu_ops.set_pte_at = lguest_set_pte_at;
pv_mmu_ops.set_pmd = lguest_set_pmd;
pv_mmu_ops.read_cr2 = lguest_read_cr2;
pv_mmu_ops.read_cr3 = lguest_read_cr3;
pv_mmu_ops.lazy_mode.enter = paravirt_enter_lazy_mmu;
pv_mmu_ops.lazy_mode.leave = lguest_leave_lazy_mode;
/* apic read/write intercepts */
pv_apic_ops.apic_write = lguest_apic_write;
pv_apic_ops.apic_write_atomic = lguest_apic_write;
pv_apic_ops.apic_read = lguest_apic_read;
/* time operations */
pv_time_ops.get_wallclock = lguest_get_wallclock;
pv_time_ops.time_init = lguest_time_init;
pv_time_ops.get_cpu_khz = lguest_cpu_khz;
/* Now is a good time to look at the implementations of these functions
* before returning to the rest of lguest_init(). */
/*G:070 Now we've seen all the paravirt_ops, we return to
* lguest_init() where the rest of the fairly chaotic boot setup
* occurs. */
/* The native boot code sets up initial page tables immediately after
* the kernel itself, and sets init_pg_tables_end so they're not
* clobbered. The Launcher places our initial pagetables somewhere at
* the top of our physical memory, so we don't need extra space: set
* init_pg_tables_end to the end of the kernel. */
init_pg_tables_end = __pa(pg0);
/* Load the %fs segment register (the per-cpu segment register) with
* the normal data segment to get through booting. */
asm volatile ("mov %0, %%fs" : : "r" (__KERNEL_DS) : "memory");
/* The Host<->Guest Switcher lives at the top of our address space, and
* the Host told us how big it is when we made LGUEST_INIT hypercall:
* it put the answer in lguest_data.reserve_mem */
/* If we don't initialize the lock dependency checker now, it crashes
* paravirt_disable_iospace. */
/* The IDE code spends about 3 seconds probing for disks: if we reserve
* all the I/O ports up front it can't get them and so doesn't probe.
* Other device drivers are similar (but less severe). This cuts the
* kernel boot time on my machine from 4.1 seconds to 0.45 seconds. */
/* This is messy CPU setup stuff which the native boot code does before
* start_kernel, so we have to do, too: */
/* head.S usually sets up the first capability word, so do it here. */
new_cpu_data.x86_capability[0] = cpuid_edx(1);
/* Math is always hard! */
new_cpu_data.hard_math = 1;
/* We don't have features. We have puppies! Puppies! */
#ifdef CONFIG_X86_MCE
mce_disabled = 1;
acpi_disabled = 1;
acpi_ht = 0;
/* We set the perferred console to "hvc". This is the "hypervisor
* virtual console" driver written by the PowerPC people, which we also
* adapted for lguest's use. */
add_preferred_console("hvc", 0, NULL);
/* Register our very early console. */
/* Last of all, we set the power management poweroff hook to point to
* the Guest routine to power off, and the reboot hook to our restart
* routine. */
pm_power_off = lguest_power_off;
machine_ops.restart = lguest_restart;
/* Now we're set up, call start_kernel() in init/main.c and we proceed
* to boot as normal. It never returns. */
* This marks the end of stage II of our journey, The Guest.
* It is now time for us to explore the layer of virtual drivers and complete
* our understanding of the Guest in "make Drivers".