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authorDenys Vlasenko <dvlasenk@redhat.com>2015-03-24 11:51:39 +1030
committerRusty Russell <rusty@rustcorp.com.au>2015-03-24 11:52:08 +1030
commit7042cb4eb30967b5eb9eeba04907882f04d6b6e5 (patch)
tree3a7412f92a18e518395da0627dc7565b3a625f58 /arch/x86/lguest
parent41f055d49cb04d648a89a2cb6d57c94ff86d5bb6 (diff)
downloadlinux-7042cb4eb30967b5eb9eeba04907882f04d6b6e5.tar.bz2
lguest: simplify lguest_iret
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com> CC: lguest@lists.ozlabs.org CC: x86@kernel.org CC: linux-kernel@vger.kernel.org Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Diffstat (limited to 'arch/x86/lguest')
-rw-r--r--arch/x86/lguest/head_32.S19
1 files changed, 9 insertions, 10 deletions
diff --git a/arch/x86/lguest/head_32.S b/arch/x86/lguest/head_32.S
index 05b0a85507ce..81678bf0fcb7 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: