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