[PATCH 2/5] x86/entry: avoid calling into sync_regs() when entering from userspace

[Lai Jiangshan <laijs@linux.alibaba.com>]

7f2590a110b8("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")
made a change that when any exception happens on userspace, the
entry code will save the pt_regs on the sp0 stack, and then copy it
to the thread stack via sync_regs() and switch to thread stack
afterward.

This is hot path, such overhead should be avoided. This patch
borrows the way how interrupt_entry handles it.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/entry/entry_64.S | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 5e983506f82e..e8817ae31390 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1321,19 +1321,13 @@ SYM_CODE_END(paranoid_exit)
 SYM_CODE_START_LOCAL(error_entry)
 	UNWIND_HINT_FUNC
 	cld
-	PUSH_AND_CLEAR_REGS save_ret=1
-	ENCODE_FRAME_POINTER 8
-	testb	$3, CS+8(%rsp)
+	testb	$3, CS-ORIG_RAX+8(%rsp)
 	jz	.Lerror_kernelspace
 
-	/*
-	 * We entered from user mode or we're pretending to have entered
-	 * from user mode due to an IRET fault.
-	 */
-	SWAPGS
-	FENCE_SWAPGS_USER_ENTRY
-	/* We have user CR3.  Change to kernel CR3. */
-	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
+	idtentry_swapgs_and_switch_to_kernel_stack
+	PUSH_AND_CLEAR_REGS save_ret=1
+	ENCODE_FRAME_POINTER 8
+	ret
 
 .Lerror_entry_from_usermode_after_swapgs:
 	/* Put us onto the real thread stack. */
@@ -1357,6 +1351,8 @@ SYM_CODE_START_LOCAL(error_entry)
 	 * for these here too.
 	 */
 .Lerror_kernelspace:
+	PUSH_AND_CLEAR_REGS save_ret=1
+	ENCODE_FRAME_POINTER 8
 	leaq	native_irq_return_iret(%rip), %rcx
 	cmpq	%rcx, RIP+8(%rsp)
 	je	.Lerror_bad_iret
-- 
2.20.1