[RFC PATCH 4/4] x86/entry/nmi: unmask NMIs on userspace NMI when entry debugging

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/entry/entry_64.S | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index f54e06139d4b..309e63f4f391 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1055,6 +1055,24 @@ SYM_CODE_START_LOCAL(error_return)
 	jmp	swapgs_restore_regs_and_return_to_usermode
 SYM_CODE_END(error_return)
 
+.macro debug_entry_unmask_NMIs
+#ifdef CONFIG_DEBUG_ENTRY
+	/*
+	 * For ease of testing, unmask NMIs right away.  Disabled by
+	 * default because IRET is very expensive.
+	 */
+	pushq	$0		/* SS */
+	pushq	%rsp		/* RSP (minus 8 because of the previous push) */
+	addq	$8, (%rsp)	/* Fix up RSP */
+	pushfq			/* RFLAGS */
+	pushq	$__KERNEL_CS	/* CS */
+	pushq	$1f		/* RIP */
+	iretq			/* continues with NMI unmasked */
+	UNWIND_HINT_IRET_REGS
+1:
+#endif
+.endm
+
 /*
  * Runs on exception stack.  Xen PV does not go through this path at all,
  * so we can use real assembly here.
@@ -1145,6 +1163,7 @@ SYM_CODE_START(asm_exc_nmi)
 	 * At this point we no longer need to worry about stack damage
 	 * due to nesting -- we're done with the NMI stack.
 	 */
+	debug_entry_unmask_NMIs
 	pushq	$-1		/* pt_regs->orig_ax */
 	idtentry_body exc_nmi has_error_code=0
 
@@ -1286,22 +1305,7 @@ first_nmi:
 	UNWIND_HINT_IRET_REGS
 
 	/* Everything up to here is safe from nested NMIs */
-
-#ifdef CONFIG_DEBUG_ENTRY
-	/*
-	 * For ease of testing, unmask NMIs right away.  Disabled by
-	 * default because IRET is very expensive.
-	 */
-	pushq	$0		/* SS */
-	pushq	%rsp		/* RSP (minus 8 because of the previous push) */
-	addq	$8, (%rsp)	/* Fix up RSP */
-	pushfq			/* RFLAGS */
-	pushq	$__KERNEL_CS	/* CS */
-	pushq	$1f		/* RIP */
-	iretq			/* continues at repeat_nmi below */
-	UNWIND_HINT_IRET_REGS
-1:
-#endif
+	debug_entry_unmask_NMIs
 
 repeat_nmi:
 	/*
-- 
2.19.1.6.gb485710b