[RFC PATCH 2/4] x86/entry/nmi: Use normal idtentry macro for NMI from userspace

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

Before tglx made huge refactor on entry code, high level code is called
from ASM code including idtentry exit path which might reopen IRQ,
reschedule, do signal among other works and made normal entry path not
suitable for userspace NMI entry.  So when the commit 9b6e6a8334d56
("x86/nmi/64: Switch stacks on userspace NMI entry") added special code
for userspace NMI entry, it didn't use normal entry code.

After the said refactor on entry code, high level code was moved into
C code, and the idtentry macros are really low level and fit for
userspace NMI entry after it switches to entry stack, so this
patch uses idtentry_body macro for NMI from userspace.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S | 42 ++++++---------------------------------
 1 file changed, 6 insertions(+), 36 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 4190e668f346..f54e06139d4b 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1113,11 +1113,7 @@ SYM_CODE_START(asm_exc_nmi)
 	jz	.Lnmi_from_kernel
 
 	/*
-	 * NMI from user mode.  We need to run on the thread stack, but we
-	 * can't go through the normal entry paths: NMIs are masked, and
-	 * we don't want to enable interrupts, because then we'll end
-	 * up in an awkward situation in which IRQs are on but NMIs
-	 * are off.
+	 * NMI from user mode.  We need to run on the thread stack.
 	 *
 	 * We also must not push anything to the stack before switching
 	 * stacks lest we corrupt the "NMI executing" variable.
@@ -1137,46 +1133,20 @@ SYM_CODE_START(asm_exc_nmi)
 		  +SIZEOF_entry_stack	/* to entry stack top */	\
 		), %rsp
 
-	/* Stash exception frame and %rdx to entry stack */
+	/* Stash exception frame and restore %rdx */
 	pushq	5*8(%rdx)	/* pt_regs->ss */
 	pushq	4*8(%rdx)	/* pt_regs->rsp */
 	pushq	3*8(%rdx)	/* pt_regs->flags */
 	pushq	2*8(%rdx)	/* pt_regs->cs */
 	pushq	1*8(%rdx)	/* pt_regs->rip */
-	pushq	0*8(%rdx)	/* %rdx */
-
-	swapgs
-	cld
-	FENCE_SWAPGS_USER_ENTRY
-	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdx
-	movq	%rsp, %rdx
-	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
-	UNWIND_HINT_IRET_REGS base=%rdx offset=8
-	pushq	5*8(%rdx)	/* pt_regs->ss */
-	pushq	4*8(%rdx)	/* pt_regs->rsp */
-	pushq	3*8(%rdx)	/* pt_regs->flags */
-	pushq	2*8(%rdx)	/* pt_regs->cs */
-	pushq	1*8(%rdx)	/* pt_regs->rip */
-	UNWIND_HINT_IRET_REGS
-	pushq   $-1		/* pt_regs->orig_ax */
-	PUSH_AND_CLEAR_REGS rdx=(%rdx)
-	ENCODE_FRAME_POINTER
+	movq	0*8(%rdx), %rdx	/* %rdx */
 
 	/*
 	 * At this point we no longer need to worry about stack damage
-	 * due to nesting -- we're on the normal thread stack and we're
-	 * done with the NMI stack.
-	 */
-
-	movq	%rsp, %rdi
-	movq	$-1, %rsi
-	call	exc_nmi
-
-	/*
-	 * Return back to user mode.  We must *not* do the normal exit
-	 * work, because we don't want to enable interrupts.
+	 * due to nesting -- we're done with the NMI stack.
 	 */
-	jmp	swapgs_restore_regs_and_return_to_usermode
+	pushq	$-1		/* pt_regs->orig_ax */
+	idtentry_body exc_nmi has_error_code=0
 
 .Lnmi_from_kernel:
 	/*
-- 
2.19.1.6.gb485710b