arch/x86/lguest/i386_head.S

   1 #include <linux/linkage.h>
   2 #include <linux/lguest.h>
   3 #include <asm/asm-offsets.h>
   4 #include <asm/thread_info.h>
   5 #include <asm/processor-flags.h>
   6
   7 /*G:020 This is where we begin: we have a magic signature which the launcher
   8  * looks for.  The plan is that the Linux boot protocol will be extended with a
   9  * "platform type" field which will guide us here from the normal entry point,
  10  * but for the moment this suffices.  The normal boot code uses %esi for the
  11  * boot header, so we do too.  We convert it to a virtual address by adding
  12  * PAGE_OFFSET, and hand it to lguest_init() as its argument (ie. %eax).
  13  *
  14  * The .section line puts this code in .init.text so it will be discarded after
  15  * boot. */
  16 .section .init.text, "ax", @progbits
  17 .ascii "GenuineLguest"
  18         /* Set up initial stack. */
  19         movl $(init_thread_union+THREAD_SIZE),%esp
  20         movl %esi, %eax
  21         addl $__PAGE_OFFSET, %eax
  22         jmp lguest_init
  23
  24 /*G:055 We create a macro which puts the assembler code between lgstart_ and
  25  * lgend_ markers.  These templates are put in the .text section: they can't be
  26  * discarded after boot as we may need to patch modules, too. */
  27 .text
  28 #define LGUEST_PATCH(name, insns...)                    \
  29         lgstart_##name: insns; lgend_##name:;           \
  30         .globl lgstart_##name; .globl lgend_##name
  31
  32 LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled)
  33 LGUEST_PATCH(sti, movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled)
  34 LGUEST_PATCH(popf, movl %eax, lguest_data+LGUEST_DATA_irq_enabled)
  35 LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
  36 /*:*/
  37
  38 /* These demark the EIP range where host should never deliver interrupts. */
  39 .global lguest_noirq_start
  40 .global lguest_noirq_end
  41
  42 /*M:004 When the Host reflects a trap or injects an interrupt into the Guest,
  43  * it sets the eflags interrupt bit on the stack based on
  44  * lguest_data.irq_enabled, so the Guest iret logic does the right thing when
  45  * restoring it.  However, when the Host sets the Guest up for direct traps,
  46  * such as system calls, the processor is the one to push eflags onto the
  47  * stack, and the interrupt bit will be 1 (in reality, interrupts are always
  48  * enabled in the Guest).
  49  *
  50  * This turns out to be harmless: the only trap which should happen under Linux
  51  * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
  52  * regions), which has to be reflected through the Host anyway.  If another
  53  * trap *does* go off when interrupts are disabled, the Guest will panic, and
  54  * we'll never get to this iret! :*/
  55
  56 /*G:045 There is one final paravirt_op that the Guest implements, and glancing
  57  * at it you can see why I left it to last.  It's *cool*!  It's in *assembler*!
  58  *
  59  * The "iret" instruction is used to return from an interrupt or trap.  The
  60  * stack looks like this:
  61  *   old address
  62  *   old code segment & privilege level
  63  *   old processor flags ("eflags")
  64  *
  65  * The "iret" instruction pops those values off the stack and restores them all
  66  * at once.  The only problem is that eflags includes the Interrupt Flag which
  67  * the Guest can't change: the CPU will simply ignore it when we do an "iret".
  68  * So we have to copy eflags from the stack to lguest_data.irq_enabled before
  69  * we do the "iret".
  70  *
  71  * There are two problems with this: firstly, we need to use a register to do
  72  * the copy and secondly, the whole thing needs to be atomic.  The first
  73  * problem is easy to solve: push %eax on the stack so we can use it, and then
  74  * restore it at the end just before the real "iret".
  75  *
  76  * The second is harder: copying eflags to lguest_data.irq_enabled will turn
  77  * interrupts on before we're finished, so we could be interrupted before we
  78  * return to userspace or wherever.  Our solution to this is to surround the
  79  * code with lguest_noirq_start: and lguest_noirq_end: labels.  We tell the
  80  * Host that it is *never* to interrupt us there, even if interrupts seem to be
  81  * enabled. */
  82 ENTRY(lguest_iret)
  83         pushl   %eax
  84         movl    12(%esp), %eax
  85 lguest_noirq_start:
  86         /* Note the %ss: segment prefix here.  Normal data accesses use the
  87          * "ds" segment, but that will have already been restored for whatever
  88          * we're returning to (such as userspace): we can't trust it.  The %ss:
  89          * prefix makes sure we use the stack segment, which is still valid. */
  90         movl    %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled
  91         popl    %eax
  92         iret
  93 lguest_noirq_end: