[PATCH 10/18] x86/asm/32: Pull MSR_IA32_SYSENTER_CS update code out of native_load_sp0()

Andy Lutomirski
2017-10-26T08:27:39Z

This causees the MSR_IA32_SYSENTER_CS write to move out of the
paravirt hook. This shouldn't affect Xen PV: Xen already ignores
MSR_IA32_SYSENTER_ESP writes. In any event, Xen doesn't support
vm86() in a useful way.

Note to any potential backporters: This patch won't break lguest, as
lguest didn't have any SYSENTER support at all.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
arch/x86/include/asm/processor.h | 7 -------
arch/x86/include/asm/switch_to.h | 11 +++++++++++
arch/x86/kernel/process_32.c | 1 +
arch/x86/kernel/process_64.c | 2 +-
arch/x86/kernel/vm86_32.c | 6 +++++-
5 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index b390ff76e58f..0167e3e35a57 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -520,13 +520,6 @@ static inline void
native_load_sp0(struct tss_struct *tss, struct thread_struct *thread)
{
tss->x86_tss.sp0 = thread->sp0;
-#ifdef CONFIG_X86_32
- /* Only happens when SEP is enabled, no need to test "SEP"arately: */
- if (unlikely(tss->x86_tss.ss1 != thread->sysenter_cs)) {
- tss->x86_tss.ss1 = thread->sysenter_cs;
- wrmsr(MSR_IA32_SYSENTER_CS, thread->sysenter_cs, 0);
- }
-#endif
}

static inline void native_swapgs(void)
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index fcc5cd387fd1..f3fa19925ae1 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -72,4 +72,15 @@ do { \
((last) = __switch_to_asm((prev), (next))); \
} while (0)

+#ifdef CONFIG_X86_32
+static inline void refresh_sysenter_cs(struct thread_struct *thread)
+{
+ /* Only happens when SEP is enabled, no need to test "SEP"arately: */
+ if (unlikely(this_cpu_read(cpu_tss.x86_tss.ss1) == thread->sysenter_cs))
+ return;
+
+ this_cpu_write(cpu_tss.x86_tss.ss1, thread->sysenter_cs);
+}
+#endif
+
#endif /* _ASM_X86_SWITCH_TO_H */
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 11966251cd42..84d6c9f554d0 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -287,6 +287,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
* current_thread_info().
*/
load_sp0(tss, next);
+ refresh_sysenter_cs(next); /* in case prev or next is vm86 */
this_cpu_write(cpu_current_top_of_stack,
(unsigned long)task_stack_page(next_p) +
THREAD_SIZE);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 302e7b2572d1..a6ff6d1a0110 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -464,7 +464,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
*/
this_cpu_write(current_task, next_p);

- /* Reload esp0 and ss1. This changes current_thread_info(). */
+ /* Reload sp0. */
load_sp0(tss, next);

/*
diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index 7924a5356c8a..5bc1c3ab6287 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -54,6 +54,7 @@
#include <asm/irq.h>
#include <asm/traps.h>
#include <asm/vm86.h>
+#include <asm/switch_to.h>

/*
* Known problems:
@@ -149,6 +150,7 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
tsk->thread.sp0 = vm86->saved_sp0;
tsk->thread.sysenter_cs = __KERNEL_CS;
load_sp0(tss, &tsk->thread);
+ refresh_sysenter_cs(&tsk->thread);
vm86->saved_sp0 = 0;
put_cpu();

@@ -368,8 +370,10 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
/* make room for real-mode segments */
tsk->thread.sp0 += 16;

- if (static_cpu_has(X86_FEATURE_SEP))
+ if (static_cpu_has(X86_FEATURE_SEP)) {
tsk->thread.sysenter_cs = 0;
+ refresh_sysenter_cs(&tsk->thread);
+ }

load_sp0(tss, &tsk->thread);
put_cpu();
--
2.13.6