Commit 7042cb4e authored by Denys Vlasenko's avatar Denys Vlasenko Committed by Rusty Russell

lguest: simplify lguest_iret

Signed-off-by: default avatarDenys Vlasenko <dvlasenk@redhat.com>
CC: lguest@lists.ozlabs.org
CC: x86@kernel.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
parent 41f055d4
......@@ -168,29 +168,28 @@ ENTRY(lg_restore_fl)
* 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:
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