x86: only enable interrupts when kernel state has been set up

The sysenter path tries to enable interrupts immediately. Unfortunately this doesn't work in a paravirt environment, because not enough kernel state has been set up at that point (namely, pointing %fs to the kernel percpu data segment). To fix this, defer ENABLE_INTERRUPTS until after the kernel state has been set up.

Unfortunately this means that we're running with interrupts disabled for a while without calling the IRQ tracing code, but that can't be called without setting up %fs either.

[ Fix since last posting: make sure saved eflags has interrupts enabled. ]

---
arch/x86/kernel/entry_32.S | 19 ++++++++++---------

1 file changed, 10 insertions(+), 9 deletions(-)

===================================================================

@@ -290,10 +290,10 @@ ENTRY(ia32_sysenter_target)

movl TSS_sysenter_sp0(%esp),%esp
ENTRY(sysenter_past_esp)

	/*

- * No need to follow this irqs on/off section: the syscall
- * disabled irqs and here we enable it straight after entry:
+ * Interrupts are disabled here, but we can't trace it until
+ * enough kernel state to call TRACE_IRQS_OFF can be called - but
+ * we immediately enable interrupts at that point anyway.

	 */
-	ENABLE_INTERRUPTS(CLBR_NONE)
	pushl $(__USER_DS)
	CFI_ADJUST_CFA_OFFSET 4
	/*CFI_REL_OFFSET ss, 0*/

@@ -301,6 +301,7 @@ ENTRY(sysenter_past_esp)
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET esp, 0
pushfl
+ orl $X86_EFLAGS_IF, (%esp)

	CFI_ADJUST_CFA_OFFSET 4
	pushl $(__USER_CS)
	CFI_ADJUST_CFA_OFFSET 4

@@ -314,6 +315,11 @@ ENTRY(sysenter_past_esp)
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET eip, 0

+ pushl %eax
+ CFI_ADJUST_CFA_OFFSET 4
+ SAVE_ALL
+ ENABLE_INTERRUPTS(CLBR_NONE)
+
/*
* Load the potential sixth argument from user stack.
* Careful about security.
@@ -321,14 +327,12 @@ ENTRY(sysenter_past_esp)

	cmpl $__PAGE_OFFSET-3,%ebp
	jae syscall_fault

1: movl (%ebp),%ebp
+ movl %ebp,PT_EBP(%esp)
.section __ex_table,"a"
.align 4
.long 1b,syscall_fault
.previous

- pushl %eax
- CFI_ADJUST_CFA_OFFSET 4
- SAVE_ALL

	GET_THREAD_INFO(%ebp)

	/* Note, _TIF_SECCOMP is bit number 8, and so it needs testw and not testb */

@@ -542,9 +546,6 @@ END(syscall_exit_work)

RING0_INT_FRAME # can't unwind into user space anyway
syscall_fault:
- pushl %eax # save orig_eax
- CFI_ADJUST_CFA_OFFSET 4
- SAVE_ALL

	GET_THREAD_INFO(%ebp)
	movl $-EFAULT,PT_EAX(%esp)
	jmp resume_userspace

thanks Jeremy - will give it a shot.

Ingo