Signed-off-by: Denys Vlasenko <dvlas...@redhat.com>
CC: lgu...@lists.ozlabs.org
CC: x...@kernel.org
CC: linux-kernel@vger.kernel.org
---
arch/x86/lguest/head_32.S | 19 +++++++++----------
1 file changed, 9 insertions(+), 10 deletions(-)
diff --git a/arch/x86/lguest/head_32.S b/arch/x86/lguest/head_32.S
index 6ddfe4f..583732c 100644
--- a/arch/x86/lguest/head_32.S
+++ b/arch/x86/lguest/head_32.S
@@ -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:
--
1.8.1.4
--
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