linux/arch/x86/xen/xen-asm_32.S

/*
	Asm versions of Xen pv-ops, suitable for either direct use or inlining.
	The inline versions are the same as the direct-use versions, with the
	pre- and post-amble chopped off.

	This code is encoded for size rather than absolute efficiency,
	with a view to being able to inline as much as possible.

	We only bother with direct forms (ie, vcpu in pda) of the operations
	here; the indirect forms are better handled in C, since they're
	generally too large to inline anyway.
 */

//#include <asm/asm-offsets.h>
#include <asm/thread_info.h>
#include <asm/processor-flags.h>
#include <asm/segment.h>

#include <xen/interface/xen.h>

#include "xen-asm.h"

/*
	Force an event check by making a hypercall,
	but preserve regs before making the call.
 */
check_events:
	push %eax
	push %ecx
	push %edx
	call xen_force_evtchn_callback
	pop %edx
	pop %ecx
	pop %eax
	ret

/*
	We can't use sysexit directly, because we're not running in ring0.
	But we can easily fake it up using iret.  Assuming xen_sysexit
	is jumped to with a standard stack frame, we can just strip it
	back to a standard iret frame and use iret.
 */
ENTRY(xen_sysexit)
	movl PT_EAX(%esp), %eax			/* Shouldn't be necessary? */
	orl $X86_EFLAGS_IF, PT_EFLAGS(%esp)
	lea PT_EIP(%esp), %esp

	jmp xen_iret
ENDPROC(xen_sysexit)

/*
	This is run where a normal iret would be run, with the same stack setup:
	      8: eflags
	      4: cs
	esp-> 0: eip

	This attempts to make sure that any pending events are dealt
	with on return to usermode, but there is a small window in
	which an event can happen just before entering usermode.  If
	the nested interrupt ends up setting one of the TIF_WORK_MASK
	pending work flags, they will not be tested again before
	returning to usermode. This means that a process can end up
	with pending work, which will be unprocessed until the process
	enters and leaves the kernel again, which could be an
	unbounded amount of time.  This means that a pending signal or
	reschedule event could be indefinitely delayed.

	The fix is to notice a nested interrupt in the critical
	window, and if one occurs, then fold the nested interrupt into
	the current interrupt stack frame, and re-process it
	iteratively rather than recursively.  This means that it will
	exit via the normal path, and all pending work will be dealt
	with appropriately.

	Because the nested interrupt handler needs to deal with the
	current stack state in whatever form its in, we keep things
	simple by only using a single register which is pushed/popped
	on the stack.
 */
ENTRY(xen_iret)
	/* test eflags for special cases */
	testl $(X86_EFLAGS_VM | XEN_EFLAGS_NMI), 8(%esp)
	jnz hyper_iret

	push %eax
	ESP_OFFSET=4	# bytes pushed onto stack

	/* Store vcpu_info pointer for easy access.  Do it this
	   way to avoid having to reload %fs */
#ifdef CONFIG_SMP
	GET_THREAD_INFO(%eax)
	movl TI_cpu(%eax),%eax
	movl __per_cpu_offset(,%eax,4),%eax
	mov per_cpu__xen_vcpu(%eax),%eax
#else
	movl per_cpu__xen_vcpu, %eax
#endif

	/* check IF state we're restoring */
	testb $X86_EFLAGS_IF>>8, 8+1+ESP_OFFSET(%esp)

	/* Maybe enable events.  Once this happens we could get a
	   recursive event, so the critical region starts immediately
	   afterwards.  However, if that happens we don't end up
	   resuming the code, so we don't have to be worried about
	   being preempted to another CPU. */
	setz XEN_vcpu_info_mask(%eax)
xen_iret_start_crit:

	/* check for unmasked and pending */
	cmpw $0x0001, XEN_vcpu_info_pending(%eax)

	/* If there's something pending, mask events again so we
	   can jump back into xen_hypervisor_callback */
	sete XEN_vcpu_info_mask(%eax)

	popl %eax

	/* From this point on the registers are restored and the stack
	   updated, so we don't need to worry about it if we're preempted */
iret_restore_end:

	/* Jump to hypervisor_callback after fixing up the stack.
	   Events are masked, so jumping out of the critical
	   region is OK. */
	je xen_hypervisor_callback

1:	iret
xen_iret_end_crit:
.section __ex_table,"a"
	.align 4
	.long 1b,iret_exc
.previous

hyper_iret:
	/* put this out of line since its very rarely used */
	jmp hypercall_page + __HYPERVISOR_iret * 32

	.globl xen_iret_start_crit, xen_iret_end_crit

/*
   This is called by xen_hypervisor_callback in entry.S when it sees
   that the EIP at the time of interrupt was between xen_iret_start_crit
   and xen_iret_end_crit.  We're passed the EIP in %eax so we can do
   a more refined determination of what to do.

   The stack format at this point is:
	----------------
	 ss		: (ss/esp may be present if we came from usermode)
	 esp		:
	 eflags		}  outer exception info
	 cs		}
	 eip		}
	---------------- <- edi (copy dest)
	 eax		:  outer eax if it hasn't been restored
	----------------
	 eflags		}  nested exception info
	 cs		}   (no ss/esp because we're nested
	 eip		}    from the same ring)
	 orig_eax	}<- esi (copy src)
	 - - - - - - - -
	 fs		}
	 es		}
	 ds		}  SAVE_ALL state
	 eax		}
	  :		:
	 ebx		}<- esp
	----------------

   In order to deliver the nested exception properly, we need to shift
   everything from the return addr up to the error code so it
   sits just under the outer exception info.  This means that when we
   handle the exception, we do it in the context of the outer exception
   rather than starting a new one.

   The only caveat is that if the outer eax hasn't been
   restored yet (ie, it's still on stack), we need to insert
   its value into the SAVE_ALL state before going on, since
   it's usermode state which we eventually need to restore.
 */
ENTRY(xen_iret_crit_fixup)
	/*
	   Paranoia: Make sure we're really coming from kernel space.
	   One could imagine a case where userspace jumps into the
	   critical range address, but just before the CPU delivers a GP,
	   it decides to deliver an interrupt instead.  Unlikely?
	   Definitely.  Easy to avoid?  Yes.  The Intel documents
	   explicitly say that the reported EIP for a bad jump is the
	   jump instruction itself, not the destination, but some virtual
	   environments get this wrong.
	 */
	movl PT_CS(%esp), %ecx
	andl $SEGMENT_RPL_MASK, %ecx
	cmpl $USER_RPL, %ecx
	je 2f

	lea PT_ORIG_EAX(%esp), %esi
	lea PT_EFLAGS(%esp), %edi

	/* If eip is before iret_restore_end then stack
	   hasn't been restored yet. */
	cmp $iret_restore_end, %eax
	jae 1f

	movl 0+4(%edi),%eax		/* copy EAX (just above top of frame) */
	movl %eax, PT_EAX(%esp)

	lea ESP_OFFSET(%edi),%edi	/* move dest up over saved regs */

	/* set up the copy */
1:	std
	mov $PT_EIP / 4, %ecx		/* saved regs up to orig_eax */
	rep movsl
	cld

	lea 4(%edi),%esp		/* point esp to new frame */
2:	jmp xen_do_upcall