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lguest: documentation update

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Went through the documentation doing typo and content fixes.  This
patch contains only comment and whitespace changes.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell
2007-10-25 15:02:50 +10:00
parent 568a17ffce
commit e1e72965ec
16 changed files with 414 additions and 266 deletions
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71
drivers/lguest/x86/switcher_32.S
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@@ -6,6 +6,37 @@
* are feeling invigorated and refreshed then the next, more challenging stage
* can be found in "make Guest". :*/
/*M:012 Lguest is meant to be simple: my rule of thumb is that 1% more LOC must
* gain at least 1% more performance. Since neither LOC nor performance can be
* measured beforehand, it generally means implementing a feature then deciding
* if it's worth it. And once it's implemented, who can say no?
*
* This is why I haven't implemented this idea myself. I want to, but I
* haven't. You could, though.
*
* The main place where lguest performance sucks is Guest page faulting. When
* a Guest userspace process hits an unmapped page we switch back to the Host,
* walk the page tables, find it's not mapped, switch back to the Guest page
* fault handler, which calls a hypercall to set the page table entry, then
* finally returns to userspace. That's two round-trips.
*
* If we had a small walker in the Switcher, we could quickly check the Guest
* page table and if the page isn't mapped, immediately reflect the fault back
* into the Guest. This means the Switcher would have to know the top of the
* Guest page table and the page fault handler address.
*
* For simplicity, the Guest should only handle the case where the privilege
* level of the fault is 3 and probably only not present or write faults. It
* should also detect recursive faults, and hand the original fault to the
* Host (which is actually really easy).
*
* Two questions remain. Would the performance gain outweigh the complexity?
* And who would write the verse documenting it? :*/
/*M:011 Lguest64 handles NMI. This gave me NMI envy (until I looked at their
* code). It's worth doing though, since it would let us use oprofile in the
* Host when a Guest is running. :*/
/*S:100
* Welcome to the Switcher itself!
*
@@ -88,7 +119,7 @@ ENTRY(switch_to_guest)
// All saved and there's now five steps before us:
// Stack, GDT, IDT, TSS
// And last of all the page tables are flipped.
// Then last of all the page tables are flipped.
// Yet beware that our stack pointer must be
// Always valid lest an NMI hits
@@ -103,25 +134,25 @@ ENTRY(switch_to_guest)
lgdt LGUEST_PAGES_guest_gdt_desc(%eax)
// The Guest's IDT we did partially
// Move to the "struct lguest_pages" as well.
// Copy to "struct lguest_pages" as well.
lidt LGUEST_PAGES_guest_idt_desc(%eax)
// The TSS entry which controls traps
// Must be loaded up with "ltr" now:
// The GDT entry that TSS uses
// Changes type when we load it: damn Intel!
// For after we switch over our page tables
// It (as the rest) will be writable no more.
// (The GDT entry TSS needs
// Changes type when we load it: damn Intel!)
// That entry will be read-only: we'd crash.
movl $(GDT_ENTRY_TSS*8), %edx
ltr %dx
// Look back now, before we take this last step!
// The Host's TSS entry was also marked used;
// Let's clear it again, ere we return.
// Let's clear it again for our return.
// The GDT descriptor of the Host
// Points to the table after two "size" bytes
movl (LGUEST_PAGES_host_gdt_desc+2)(%eax), %edx
// Clear the type field of "used" (byte 5, bit 2)
// Clear "used" from type field (byte 5, bit 2)
andb $0xFD, (GDT_ENTRY_TSS*8 + 5)(%edx)
// Once our page table's switched, the Guest is live!
@@ -131,7 +162,7 @@ ENTRY(switch_to_guest)
// The page table change did one tricky thing:
// The Guest's register page has been mapped
// Writable onto our %esp (stack) --
// Writable under our %esp (stack) --
// We can simply pop off all Guest regs.
popl %eax
popl %ebx
@@ -152,16 +183,15 @@ ENTRY(switch_to_guest)
addl $8, %esp
// The last five stack slots hold return address
// And everything needed to change privilege
// Into the Guest privilege level of 1,
// And everything needed to switch privilege
// From Switcher's level 0 to Guest's 1,
// And the stack where the Guest had last left it.
// Interrupts are turned back on: we are Guest.
iret
// There are two paths where we switch to the Host
// We treat two paths to switch back to the Host
// Yet both must save Guest state and restore Host
// So we put the routine in a macro.
// We are on our way home, back to the Host
// Interrupted out of the Guest, we come here.
#define SWITCH_TO_HOST \
/* We save the Guest state: all registers first \
* Laid out just as "struct lguest_regs" defines */ \
@@ -194,7 +224,7 @@ ENTRY(switch_to_guest)
movl %esp, %eax; \
andl $(~(1 << PAGE_SHIFT - 1)), %eax; \
/* Save our trap number: the switch will obscure it \
* (The Guest regs are not mapped here in the Host) \
* (In the Host the Guest regs are not mapped here) \
* %ebx holds it safe for deliver_to_host */ \
movl LGUEST_PAGES_regs_trapnum(%eax), %ebx; \
/* The Host GDT, IDT and stack! \
@@ -210,9 +240,9 @@ ENTRY(switch_to_guest)
/* Switch to Host's GDT, IDT. */ \
lgdt LGUEST_PAGES_host_gdt_desc(%eax); \
lidt LGUEST_PAGES_host_idt_desc(%eax); \
/* Restore the Host's stack where it's saved regs lie */ \
/* Restore the Host's stack where its saved regs lie */ \
movl LGUEST_PAGES_host_sp(%eax), %esp; \
/* Last the TSS: our Host is complete */ \
/* Last the TSS: our Host is returned */ \
movl $(GDT_ENTRY_TSS*8), %edx; \
ltr %dx; \
/* Restore now the regs saved right at the first. */ \
@@ -222,14 +252,15 @@ ENTRY(switch_to_guest)
popl %ds; \
popl %es
// Here's where we come when the Guest has just trapped:
// (Which trap we'll see has been pushed on the stack).
// The first path is trod when the Guest has trapped:
// (Which trap it was has been pushed on the stack).
// We need only switch back, and the Host will decode
// Why we came home, and what needs to be done.
return_to_host:
SWITCH_TO_HOST
iret
// We are lead to the second path like so:
// An interrupt, with some cause external
// Has ajerked us rudely from the Guest's code
// Again we must return home to the Host
@@ -238,7 +269,7 @@ deliver_to_host:
// But now we must go home via that place
// Where that interrupt was supposed to go
// Had we not been ensconced, running the Guest.
// Here we see the cleverness of our stack:
// Here we see the trickness of run_guest_once():
// The Host stack is formed like an interrupt
// With EIP, CS and EFLAGS layered.
// Interrupt handlers end with "iret"
@@ -263,7 +294,7 @@ deliver_to_host:
xorw %ax, %ax
orl %eax, %edx
// Now the address of the handler's in %edx
// We call it now: its "iret" takes us home.
// We call it now: its "iret" drops us home.
jmp *%edx
// Every interrupt can come to us here