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lguest: comment documentation update.

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Took some cycles to re-read the Lguest Journey end-to-end, fix some
rot and tighten some phrases.

Only comments change.  No new jokes, but a couple of recycled old jokes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell
2008-03-28 11:05:53 -05:00
parent e18b094f0f
commit a6bd8e1303
13 changed files with 208 additions and 142 deletions
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108
arch/x86/lguest/boot.c
View File

@@ -10,21 +10,19 @@
* (such as the example in Documentation/lguest/lguest.c) is called the
* Launcher.
*
* Secondly, we only run specially modified Guests, not normal kernels. When
* you set CONFIG_LGUEST to 'y' or 'm', this automatically sets
* CONFIG_LGUEST_GUEST=y, which compiles this file into the kernel so it knows
* how to be a Guest. This means that you can use the same kernel you boot
* normally (ie. as a Host) as a Guest.
* 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? The Guest starts at a special
* entry point marked with a magic string, which sets up a few things then
* calls here. We replace the native functions various "paravirt" structures
* with our Guest versions, then boot like normal. :*/
* 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 <rusty@rustcorp.com.au> IBM Corporation.
@@ -134,7 +132,7 @@ static void async_hcall(unsigned long call, unsigned long arg1,
* lguest_leave_lazy_mode().
*
* So, when we're in lazy mode, we call async_hcall() to store the call for
* future processing. */
* future processing: */
static void lazy_hcall(unsigned long call,
unsigned long arg1,
unsigned long arg2,
@@ -147,7 +145,7 @@ static void lazy_hcall(unsigned long call,
}
/* When lazy mode is turned off reset the per-cpu lazy mode variable and then
* issue a hypercall to flush any stored calls. */
* issue the do-nothing hypercall to flush any stored calls. */
static void lguest_leave_lazy_mode(void)
{
paravirt_leave_lazy(paravirt_get_lazy_mode());
@@ -164,7 +162,7 @@ static void lguest_leave_lazy_mode(void)
*
* 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 when it wants to deliver an interrupt.
* check there before it tries to deliver an interrupt.
*/
/* save_flags() is expected to return the processor state (ie. "flags"). The
@@ -196,10 +194,15 @@ static void irq_enable(void)
/*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 when we turn on CONFIG_NO_HZ, we should revisit this. One way
* 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. :*/
* 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. :*/
/*G:034
* The Interrupt Descriptor Table (IDT).
@@ -212,6 +215,10 @@ static void irq_enable(void)
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);
@@ -243,7 +250,8 @@ static void lguest_load_idt(const struct desc_ptr *desc)
*
* 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?
* 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)
{
@@ -298,9 +306,9 @@ 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 and AMD. 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.
* 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!
@@ -594,17 +602,17 @@ static unsigned long lguest_get_wallclock(void)
return lguest_data.time.tv_sec;
}
/* The TSC is a 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 not register
* itself. */
/* 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 "lguest_clock", where
* we read the time value given to us by the Host. */
/* 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;
@@ -648,12 +656,16 @@ static struct clocksource lguest_clock = {
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",
__FUNCTION__, delta);
return -ETIME;
}
/* Please wake us this far in the future. */
hcall(LHCALL_SET_CLOCKEVENT, delta, 0, 0);
return 0;
}
@@ -738,7 +750,7 @@ static void lguest_time_init(void)
* 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)
struct thread_struct *thread)
{
lazy_hcall(LHCALL_SET_STACK, __KERNEL_DS|0x1, thread->sp0,
THREAD_SIZE/PAGE_SIZE);
@@ -786,9 +798,8 @@ static void lguest_safe_halt(void)
hcall(LHCALL_HALT, 0, 0, 0);
}
/* Perhaps CRASH isn't the best name for this hypercall, but we use it to get a
* message out when we're crashing as well as elegant termination like powering
* off.
/* 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. */
@@ -816,8 +827,9 @@ static struct notifier_block paniced = {
/* Setting up memory is fairly easy. */
static __init char *lguest_memory_setup(void)
{
/* We do this here and not earlier because lockcheck barfs if we do it
* before start_kernel() */
/* 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
@@ -850,12 +862,19 @@ static __init int early_put_chars(u32 vtermno, const char *buf, int count)
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)
{
hcall(LHCALL_SHUTDOWN, __pa(reason), LGUEST_SHUTDOWN_RESTART, 0);
}
/*G:050
* 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
* 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
@@ -908,14 +927,9 @@ static unsigned lguest_patch(u8 type, u16 clobber, void *ibuf,
return insn_len;
}
static void lguest_restart(char *reason)
{
hcall(LHCALL_SHUTDOWN, __pa(reason), LGUEST_SHUTDOWN_RESTART, 0);
}
/*G:030 Once we get to lguest_init(), we know we're a Guest. The pv_ops
* structures in the kernel provide points for (almost) every routine we have
* to override to avoid privileged instructions. */
/*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
@@ -1003,9 +1017,9 @@ __init void lguest_init(void)
* the normal data segment to get through booting. */
asm volatile ("mov %0, %%fs" : : "r" (__KERNEL_DS) : "memory");
/* The Host uses the top of the Guest's virtual address space for the
* Host<->Guest Switcher, and it tells us how big that is in
* lguest_data.reserve_mem, set up on the LGUEST_INIT hypercall. */
/* 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 */
reserve_top_address(lguest_data.reserve_mem);
/* If we don't initialize the lock dependency checker now, it crashes
@@ -1027,6 +1041,7 @@ __init void lguest_init(void)
/* 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;
#endif
@@ -1044,10 +1059,11 @@ __init void lguest_init(void)
virtio_cons_early_init(early_put_chars);
/* Last of all, we set the power management poweroff hook to point to
* the Guest routine to power off. */
* 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. */
start_kernel();