diff options
| author | Denys Vlasenko <dvlasenk@redhat.com> | 2015-03-24 02:21:39 +0100 |
|---|---|---|
| committer | Rusty Russell <rusty@rustcorp.com.au> | 2015-03-24 02:22:08 +0100 |
| commit | 7042cb4eb30967b5eb9eeba04907882f04d6b6e5 (patch) | |
| tree | 3a7412f92a18e518395da0627dc7565b3a625f58 /arch/x86 | |
| parent | lguest: rename i386_head.S in the comments (diff) | |
| download | linux-7042cb4eb30967b5eb9eeba04907882f04d6b6e5.tar.xz linux-7042cb4eb30967b5eb9eeba04907882f04d6b6e5.zip | |
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')
| -rw-r--r-- | arch/x86/lguest/head_32.S | 19 |
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: |