arch/x86/lguest/head_32.S

   1 #include <linux/linkage.h>
   2 #include <linux/lguest.h>
   3 #include <asm/lguest_hcall.h>
   4 #include <asm/asm-offsets.h>
   5 #include <asm/thread_info.h>
   6 #include <asm/processor-flags.h>
   7
   8 /*G:020
   9
  10  * Our story starts with the bzImage: booting starts at startup_32 in
  11  * arch/x86/boot/compressed/head_32.S.  This merely uncompresses the real
  12  * kernel in place and then jumps into it: startup_32 in
  13  * arch/x86/kernel/head_32.S.  Both routines expects a boot header in the %esi
  14  * register, which is created by the bootloader (the Launcher in our case).
  15  *
  16  * The startup_32 function does very little: it clears the uninitialized global
  17  * C variables which we expect to be zero (ie. BSS) and then copies the boot
  18  * header and kernel command line somewhere safe, and populates some initial
  19  * page tables.  Finally it checks the 'hardware_subarch' field.  This was
  20  * introduced in 2.6.24 for lguest and Xen: if it's set to '1' (lguest's
  21  * assigned number), then it calls us here.
  22  *
  23  * WARNING: be very careful here!  We're running at addresses equal to physical
  24  * addresses (around 0), not above PAGE_OFFSET as most code expects
  25  * (eg. 0xC0000000).  Jumps are relative, so they're OK, but we can't touch any
  26  * data without remembering to subtract __PAGE_OFFSET!
  27  *
  28  * The .section line puts this code in .init.text so it will be discarded after
  29  * boot.
  30  */
  31 .section .init.text, "ax", @progbits
  32 ENTRY(lguest_entry)
  33         /*
  34          * We make the "initialization" hypercall now to tell the Host where
  35          * our lguest_data struct is.
  36          */
  37         movl $LHCALL_LGUEST_INIT, %eax
  38         movl $lguest_data - __PAGE_OFFSET, %ebx
  39         int $LGUEST_TRAP_ENTRY
  40
  41         /* Now turn our pagetables on; setup by arch/x86/kernel/head_32.S. */
  42         movl $LHCALL_NEW_PGTABLE, %eax
  43         movl $(initial_page_table - __PAGE_OFFSET), %ebx
  44         int $LGUEST_TRAP_ENTRY
  45
  46         /* Set up the initial stack so we can run C code. */
  47         movl $(init_thread_union+THREAD_SIZE),%esp
  48
  49         /* Jumps are relative: we're running __PAGE_OFFSET too low. */
  50         jmp lguest_init+__PAGE_OFFSET
  51
  52 /*G:055
  53  * We create a macro which puts the assembler code between lgstart_ and lgend_
  54  * markers.  These templates are put in the .text section: they can't be
  55  * discarded after boot as we may need to patch modules, too.
  56  */
  57 .text
  58 #define LGUEST_PATCH(name, insns...)                    \
  59         lgstart_##name: insns; lgend_##name:;           \
  60         .globl lgstart_##name; .globl lgend_##name
  61
  62 LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled)
  63 LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)
  64
  65 /*G:033
  66  * But using those wrappers is inefficient (we'll see why that doesn't matter
  67  * for save_fl and irq_disable later).  If we write our routines carefully in
  68  * assembler, we can avoid clobbering any registers and avoid jumping through
  69  * the wrapper functions.
  70  *
  71  * I skipped over our first piece of assembler, but this one is worth studying
  72  * in a bit more detail so I'll describe in easy stages.  First, the routine to
  73  * enable interrupts:
  74  */
  75 ENTRY(lg_irq_enable)
  76         /*
  77          * The reverse of irq_disable, this sets lguest_data.irq_enabled to
  78          * X86_EFLAGS_IF (ie. "Interrupts enabled").
  79          */
  80         movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled
  81         /*
  82          * But now we need to check if the Host wants to know: there might have
  83          * been interrupts waiting to be delivered, in which case it will have
  84          * set lguest_data.irq_pending to X86_EFLAGS_IF.  If it's not zero, we
  85          * jump to send_interrupts, otherwise we're done.
  86          */
  87         testl $0, lguest_data+LGUEST_DATA_irq_pending
  88         jnz send_interrupts
  89         /*
  90          * One cool thing about x86 is that you can do many things without using
  91          * a register.  In this case, the normal path hasn't needed to save or
  92          * restore any registers at all!
  93          */
  94         ret
  95 send_interrupts:
  96         /*
  97          * OK, now we need a register: eax is used for the hypercall number,
  98          * which is LHCALL_SEND_INTERRUPTS.
  99          *
 100          * We used not to bother with this pending detection at all, which was
 101          * much simpler.  Sooner or later the Host would realize it had to
 102          * send us an interrupt.  But that turns out to make performance 7
 103          * times worse on a simple tcp benchmark.  So now we do this the hard
 104          * way.
 105          */
 106         pushl %eax
 107         movl $LHCALL_SEND_INTERRUPTS, %eax
 108         /* This is the actual hypercall trap. */
 109         int  $LGUEST_TRAP_ENTRY
 110         /* Put eax back the way we found it. */
 111         popl %eax
 112         ret
 113
 114 /*
 115  * Finally, the "popf" or "restore flags" routine.  The %eax register holds the
 116  * flags (in practice, either X86_EFLAGS_IF or 0): if it's X86_EFLAGS_IF we're
 117  * enabling interrupts again, if it's 0 we're leaving them off.
 118  */
 119 ENTRY(lg_restore_fl)
 120         /* This is just "lguest_data.irq_enabled = flags;" */
 121         movl %eax, lguest_data+LGUEST_DATA_irq_enabled
 122         /*
 123          * Now, if the %eax value has enabled interrupts and
 124          * lguest_data.irq_pending is set, we want to tell the Host so it can
 125          * deliver any outstanding interrupts.  Fortunately, both values will
 126          * be X86_EFLAGS_IF (ie. 512) in that case, and the "testl"
 127          * instruction will AND them together for us.  If both are set, we
 128          * jump to send_interrupts.
 129          */
 130         testl lguest_data+LGUEST_DATA_irq_pending, %eax
 131         jnz send_interrupts
 132         /* Again, the normal path has used no extra registers.  Clever, huh? */
 133         ret
 134 /*:*/
 135
 136 /* These demark the EIP range where host should never deliver interrupts. */
 137 .global lguest_noirq_start
 138 .global lguest_noirq_end
 139
 140 /*M:004
 141  * When the Host reflects a trap or injects an interrupt into the Guest, it
 142  * sets the eflags interrupt bit on the stack based on lguest_data.irq_enabled,
 143  * so the Guest iret logic does the right thing when restoring it.  However,
 144  * when the Host sets the Guest up for direct traps, such as system calls, the
 145  * processor is the one to push eflags onto the stack, and the interrupt bit
 146  * will be 1 (in reality, interrupts are always enabled in the Guest).
 147  *
 148  * This turns out to be harmless: the only trap which should happen under Linux
 149  * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc
 150  * regions), which has to be reflected through the Host anyway.  If another
 151  * trap *does* go off when interrupts are disabled, the Guest will panic, and
 152  * we'll never get to this iret!
 153 :*/
 154
 155 /*G:045
 156  * There is one final paravirt_op that the Guest implements, and glancing at it
 157  * you can see why I left it to last.  It's *cool*!  It's in *assembler*!
 158  *
 159  * The "iret" instruction is used to return from an interrupt or trap.  The
 160  * stack looks like this:
 161  *   old address
 162  *   old code segment & privilege level
 163  *   old processor flags ("eflags")
 164  *
 165  * The "iret" instruction pops those values off the stack and restores them all
 166  * at once.  The only problem is that eflags includes the Interrupt Flag which
 167  * the Guest can't change: the CPU will simply ignore it when we do an "iret".
 168  * So we have to copy eflags from the stack to lguest_data.irq_enabled before
 169  * we do the "iret".
 170  *
 171  * There are two problems with this: firstly, we need to use a register to do
 172  * the copy and secondly, the whole thing needs to be atomic.  The first
 173  * problem is easy to solve: push %eax on the stack so we can use it, and then
 174  * restore it at the end just before the real "iret".
 175  *
 176  * The second is harder: copying eflags to lguest_data.irq_enabled will turn
 177  * interrupts on before we're finished, so we could be interrupted before we
 178  * return to userspace or wherever.  Our solution to this is to surround the
 179  * code with lguest_noirq_start: and lguest_noirq_end: labels.  We tell the
 180  * Host that it is *never* to interrupt us there, even if interrupts seem to be
 181  * enabled.
 182  */
 183 ENTRY(lguest_iret)
 184         pushl   %eax
 185         movl    12(%esp), %eax
 186 lguest_noirq_start:
 187         /*
 188          * Note the %ss: segment prefix here.  Normal data accesses use the
 189          * "ds" segment, but that will have already been restored for whatever
 190          * we're returning to (such as userspace): we can't trust it.  The %ss:
 191          * prefix makes sure we use the stack segment, which is still valid.
 192          */
 193         movl    %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled
 194         popl    %eax
 195         iret
 196 lguest_noirq_end: