i386: convert hardware exception 7 to an interrupt gate

Handle no coprocessor exception with interrupt initially off.

device_not_available in entry_32.S calls either math_state_restore
or math_emulate. This patch adds an extra indirection to be
able to re-enable interrupts explicitly in traps_32.c
Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/entry_32.S

@@ -760,20 +760,9 @@ ENTRY(device_not_available)
 	RING0_INT_FRAME
 	pushl $-1			# mark this as an int
 	CFI_ADJUST_CFA_OFFSET 4
-	SAVE_ALL
-	GET_CR0_INTO_EAX
-	testl $0x4, %eax		# EM (math emulation bit)
-	jne device_not_available_emulate
-	preempt_stop(CLBR_ANY)
-	call math_state_restore
-	jmp ret_from_exception
-device_not_available_emulate:
-	pushl $0			# temporary storage for ORIG_EIP
+	pushl $do_device_not_available
 	CFI_ADJUST_CFA_OFFSET 4
-	call math_emulate
-	addl $4, %esp
-	CFI_ADJUST_CFA_OFFSET -4
-	jmp ret_from_exception
+	jmp error_code
 	CFI_ENDPROC
 END(device_not_available)
 

arch/x86/kernel/traps_32.c

@@ -46,6 +46,7 @@
 #include <linux/edac.h>
 #endif
 
+#include <asm/processor-flags.h>
 #include <asm/arch_hooks.h>
 #include <asm/stacktrace.h>
 #include <asm/processor.h>
@@ -1236,6 +1237,17 @@ asmlinkage void math_emulate(long arg)
 
 #endif /* CONFIG_MATH_EMULATION */
 
+void __kprobes do_device_not_available(struct pt_regs *regs, long error)
+{
+	if (read_cr0() & X86_CR0_EM) {
+		conditional_sti(regs);
+		math_emulate(0);
+	} else {
+		math_state_restore(); /* interrupts still off */
+		conditional_sti(regs);
+	}
+}
+
 void __init trap_init(void)
 {
 	int i;
@@ -1255,7 +1267,7 @@ void __init trap_init(void)
 	set_system_intr_gate(4, &overflow); /* int4 can be called from all */
 	set_intr_gate(5, &bounds);
 	set_intr_gate(6, &invalid_op);
-	set_trap_gate(7, &device_not_available);
+	set_intr_gate(7, &device_not_available);
 	set_task_gate(8, GDT_ENTRY_DOUBLEFAULT_TSS);
 	set_trap_gate(9, &coprocessor_segment_overrun);
 	set_trap_gate(10, &invalid_TSS);