kernel-ark/arch/x86/lguest/i386_head.S
Matias Zabaljauregui 58a2456644 lguest: move the initial guest page table creation code to the host
This patch moves the initial guest page table creation code to the host,
so the launcher keeps working with PAE enabled configs.

Signed-off-by: Matias Zabaljauregui <zabaljauregui@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
2008-12-30 09:26:11 +10:30

110 lines
4.9 KiB
ArmAsm

#include <linux/linkage.h>
#include <linux/lguest.h>
#include <asm/lguest_hcall.h>
#include <asm/asm-offsets.h>
#include <asm/thread_info.h>
#include <asm/processor-flags.h>
/*G:020 Our story starts with the kernel booting into startup_32 in
* arch/x86/kernel/head_32.S. It expects a boot header, which is created by
* the bootloader (the Launcher in our case).
*
* The startup_32 function does very little: it clears the uninitialized global
* C variables which we expect to be zero (ie. BSS) and then copies the boot
* header and kernel command line somewhere safe. Finally it checks the
* 'hardware_subarch' field. This was introduced in 2.6.24 for lguest and Xen:
* if it's set to '1' (lguest's assigned number), then it calls us here.
*
* WARNING: be very careful here! We're running at addresses equal to physical
* addesses (around 0), not above PAGE_OFFSET as most code expectes
* (eg. 0xC0000000). Jumps are relative, so they're OK, but we can't touch any
* data without remembering to subtract __PAGE_OFFSET!
*
* The .section line puts this code in .init.text so it will be discarded after
* boot. */
.section .init.text, "ax", @progbits
ENTRY(lguest_entry)
/* We make the "initialization" hypercall now to tell the Host about
* us, and also find out where it put our page tables. */
movl $LHCALL_LGUEST_INIT, %eax
movl $lguest_data - __PAGE_OFFSET, %edx
int $LGUEST_TRAP_ENTRY
/* Set up the initial stack so we can run C code. */
movl $(init_thread_union+THREAD_SIZE),%esp
/* Jumps are relative, and we're running __PAGE_OFFSET too low at the
* moment. */
jmp lguest_init+__PAGE_OFFSET
/*G:055 We create a macro which puts the assembler code between lgstart_ and
* lgend_ markers. These templates are put in the .text section: they can't be
* discarded after boot as we may need to patch modules, too. */
.text
#define LGUEST_PATCH(name, insns...) \
lgstart_##name: insns; lgend_##name:; \
.globl lgstart_##name; .globl lgend_##name
LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled)
LGUEST_PATCH(sti, movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled)
LGUEST_PATCH(popf, movl %eax, lguest_data+LGUEST_DATA_irq_enabled)
LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
/*:*/
/* These demark the EIP range where host should never deliver interrupts. */
.global lguest_noirq_start
.global lguest_noirq_end
/*M:004 When the Host reflects a trap or injects an interrupt into the Guest,
* it sets the eflags interrupt bit on the stack based on
* lguest_data.irq_enabled, so the Guest iret logic does the right thing when
* restoring it. However, when the Host sets the Guest up for direct traps,
* such as system calls, the processor is the one to push eflags onto the
* stack, and the interrupt bit will be 1 (in reality, interrupts are always
* enabled in the Guest).
*
* This turns out to be harmless: the only trap which should happen under Linux
* with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
* regions), which has to be reflected through the Host anyway. If another
* trap *does* go off when interrupts are disabled, the Guest will panic, and
* we'll never get to this iret! :*/
/*G:045 There is one final paravirt_op that the Guest implements, and glancing
* at it you can see why I left it to last. It's *cool*! It's in *assembler*!
*
* The "iret" instruction is used to return from an interrupt or trap. The
* stack looks like this:
* old address
* old code segment & privilege level
* old processor flags ("eflags")
*
* The "iret" instruction pops those values off the stack and restores them all
* at once. The only problem is that eflags includes the Interrupt Flag which
* the Guest can't change: the CPU will simply ignore it when we do an "iret".
* So we have to copy eflags from the stack to lguest_data.irq_enabled before
* we do the "iret".
*
* There are two problems with this: firstly, we need to use a register to do
* the copy and secondly, the whole thing needs to be atomic. The first
* problem is easy to solve: push %eax on the stack so we can use it, and then
* restore it at the end just before the real "iret".
*
* The second is harder: copying eflags to lguest_data.irq_enabled will turn
* interrupts on before we're finished, so we could be interrupted before we
* return to userspace or wherever. Our solution to this is to surround the
* code with lguest_noirq_start: and lguest_noirq_end: labels. We tell the
* Host that it is *never* to interrupt us there, even if interrupts seem to be
* enabled. */
ENTRY(lguest_iret)
pushl %eax
movl 12(%esp), %eax
lguest_noirq_start:
/* Note the %ss: segment prefix here. Normal data accesses use the
* "ds" segment, but that will have already been restored for whatever
* we're returning to (such as userspace): we can't trust it. The %ss:
* prefix makes sure we use the stack segment, which is still valid. */
movl %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled
popl %eax
iret
lguest_noirq_end: