i386_head.S

#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, %ebx
	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: we're running __PAGE_OFFSET too low. */
	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(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax)

/*G:033
 * But using those wrappers is inefficient (we'll see why that doesn't matter
 * for save_fl and irq_disable later).  If we write our routines carefully in
 * assembler, we can avoid clobbering any registers and avoid jumping through
 * the wrapper functions.
 *
 * I skipped over our first piece of assembler, but this one is worth studying
 * in a bit more detail so I'll describe in easy stages.  First, the routine to
 * enable interrupts:
 */
ENTRY(lg_irq_enable)
	/*
	 * The reverse of irq_disable, this sets lguest_data.irq_enabled to
	 * X86_EFLAGS_IF (ie. "Interrupts enabled").
	 */
	movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled
	/*
	 * But now we need to check if the Host wants to know: there might have
	 * been interrupts waiting to be delivered, in which case it will have
	 * set lguest_data.irq_pending to X86_EFLAGS_IF.  If it's not zero, we
	 * jump to send_interrupts, otherwise we're done.
	 */
	testl $0, lguest_data+LGUEST_DATA_irq_pending
	jnz send_interrupts
	/*
	 * One cool thing about x86 is that you can do many things without using
	 * a register.  In this case, the normal path hasn't needed to save or
	 * restore any registers at all!
	 */
	ret
send_interrupts:
	/*
	 * OK, now we need a register: eax is used for the hypercall number,
	 * which is LHCALL_SEND_INTERRUPTS.
	 *
	 * We used not to bother with this pending detection at all, which was
	 * much simpler.  Sooner or later the Host would realize it had to
	 * send us an interrupt.  But that turns out to make performance 7
	 * times worse on a simple tcp benchmark.  So now we do this the hard
	 * way.
	 */
	pushl %eax
	movl $LHCALL_SEND_INTERRUPTS, %eax
	/*
	 * This is a vmcall instruction (same thing that KVM uses).  Older
	 * assembler versions might not know the "vmcall" instruction, so we
	 * create one manually here.
	 */
	.byte 0x0f,0x01,0xc1 /* KVM_HYPERCALL */
	/* Put eax back the way we found it. */
	popl %eax
	ret

/*
 * Finally, the "popf" or "restore flags" routine.  The %eax register holds the
 * flags (in practice, either X86_EFLAGS_IF or 0): if it's X86_EFLAGS_IF we're
 * enabling interrupts again, if it's 0 we're leaving them off.
 */
ENTRY(lg_restore_fl)
	/* This is just "lguest_data.irq_enabled = flags;" */
	movl %eax, lguest_data+LGUEST_DATA_irq_enabled
	/*
	 * Now, if the %eax value has enabled interrupts and
	 * lguest_data.irq_pending is set, we want to tell the Host so it can
	 * deliver any outstanding interrupts.  Fortunately, both values will
	 * be X86_EFLAGS_IF (ie. 512) in that case, and the "testl"
	 * instruction will AND them together for us.  If both are set, we
	 * jump to send_interrupts.
	 */
	testl lguest_data+LGUEST_DATA_irq_pending, %eax
	jnz send_interrupts
	/* Again, the normal path has used no extra registers.  Clever, huh? */
	ret
/*:*/

/* 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: