From 7042cb4eb30967b5eb9eeba04907882f04d6b6e5 Mon Sep 17 00:00:00 2001 From: Denys Vlasenko Date: Tue, 24 Mar 2015 11:51:39 +1030 Subject: [PATCH] lguest: simplify lguest_iret Signed-off-by: Denys Vlasenko CC: lguest@lists.ozlabs.org CC: x86@kernel.org CC: linux-kernel@vger.kernel.org Signed-off-by: Rusty Russell --- 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 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: -- 2.39.5