xen, x86/entry/64: Add xen NMI trap entry

Instead of trying to execute any NMI via the bare metal's NMI trap
handler use a Xen specific one for PV domains, like we do for e.g.
debug traps. As in a PV domain the NMI is handled via the normal
kernel stack this is the correct thing to do.

This will enable us to get rid of the very fragile and questionable
dependencies between the bare metal NMI handler and Xen assumptions
believed to be broken anyway.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/5baf5c0528d58402441550c5770b98e7961e7680.1509609304.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64.S

@@ -1078,6 +1078,7 @@ idtentry int3			do_int3			has_error_code=0	paranoid=1 shift_ist=DEBUG_STACK
 idtentry stack_segment		do_stack_segment	has_error_code=1
 
 #ifdef CONFIG_XEN
+idtentry xennmi			do_nmi			has_error_code=0
 idtentry xendebug		do_debug		has_error_code=0
 idtentry xenint3		do_int3			has_error_code=0
 #endif
@@ -1240,7 +1241,6 @@ ENTRY(error_exit)
 END(error_exit)
 
 /* Runs on exception stack */
-/* XXX: broken on Xen PV */
 ENTRY(nmi)
 	UNWIND_HINT_IRET_REGS
 	/*

arch/x86/include/asm/traps.h

@@ -37,9 +37,9 @@ asmlinkage void simd_coprocessor_error(void);
 
 #if defined(CONFIG_X86_64) && defined(CONFIG_XEN_PV)
 asmlinkage void xen_divide_error(void);
+asmlinkage void xen_xennmi(void);
 asmlinkage void xen_xendebug(void);
 asmlinkage void xen_xenint3(void);
-asmlinkage void xen_nmi(void);
 asmlinkage void xen_overflow(void);
 asmlinkage void xen_bounds(void);
 asmlinkage void xen_invalid_op(void);

arch/x86/xen/enlighten_pv.c

@@ -600,7 +600,7 @@ static struct trap_array_entry trap_array[] = {
 #ifdef CONFIG_X86_MCE
 	{ machine_check,               xen_machine_check,               true },
 #endif
-	{ nmi,                         xen_nmi,                         true },
+	{ nmi,                         xen_xennmi,                      true },
 	{ overflow,                    xen_overflow,                    false },
 #ifdef CONFIG_IA32_EMULATION
 	{ entry_INT80_compat,          xen_entry_INT80_compat,          false },

arch/x86/xen/xen-asm_64.S

@@ -29,7 +29,7 @@ xen_pv_trap debug
 xen_pv_trap xendebug
 xen_pv_trap int3
 xen_pv_trap xenint3
-xen_pv_trap nmi
+xen_pv_trap xennmi
 xen_pv_trap overflow
 xen_pv_trap bounds
 xen_pv_trap invalid_op