lguest: simplify lguest_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".

 * There are two problems with this: firstly, we can't clobber any registers

 * and secondly, the whole thing needs to be atomic.  The first problem

 * is solved by using "push memory"/"pop memory" instruction pair for copying.

 *

 * 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.

 * enabled. (It's not necessary to protect pop instruction, since

 * data gets updated only after it completes, so we end up surrounding

 * just one instruction, iret).

 */

ENTRY(lguest_iret)

	pushl	%eax

	movl	12(%esp), %eax

lguest_noirq_start:

	pushl	2*4(%esp)

	/*

	 * 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

	popl	%ss:lguest_data+LGUEST_DATA_irq_enabled

lguest_noirq_start:

	iret

lguest_noirq_end: