[PATCH V3 36/49] x86/entry: Implement the C version ist_paranoid_exit()

[Lai Jiangshan <jiangshanlai@gmail.com>]

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

It implements the whole ASM version paranoid_exit().

No functional difference intended.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/entry/entry64.c        | 40 +++++++++++++++++++++++++++++++++
 arch/x86/include/asm/idtentry.h |  2 ++
 2 files changed, 42 insertions(+)

diff --git a/arch/x86/entry/entry64.c b/arch/x86/entry/entry64.c
index 8e1474ece7f9..51293ecbfe3f 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -325,3 +325,43 @@ void ist_paranoid_entry(unsigned long *cr3, unsigned long *gsbase)
 	/* Handle GSBASE, store the return value in *@gsbase for exit. */
 	*gsbase = ist_switch_to_kernel_gsbase();
 }
+
+/*
+ * "Paranoid" exit path from exception stack.  This is invoked
+ * only on return from IST interrupts that came from kernel space.
+ *
+ * We may be returning to very strange contexts (e.g. very early
+ * in syscall entry), so checking for preemption here would
+ * be complicated.  Fortunately, there's no good reason to try
+ * to handle preemption here.
+ */
+__visible __entry_text
+void ist_paranoid_exit(unsigned long cr3, unsigned long gsbase)
+{
+	/*
+	 * Restore CR3 at first, it can use kernel GSBASE.
+	 */
+	ist_restore_cr3(cr3);
+
+	barrier();
+
+	/*
+	 * Handle the three GSBASE cases.
+	 *
+	 * @gsbase contains the GSBASE related information depending
+	 * on the availability of the FSGSBASE instructions:
+	 *
+	 * FSGSBASE	@gsbase
+	 *     N        0 -> SWAPGS on exit
+	 *              1 -> no SWAPGS on exit
+	 *
+	 *     Y        User space GSBASE, must be restored unconditionally
+	 */
+	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
+		wrgsbase(gsbase);
+		return;
+	}
+
+	if (gsbase)
+		native_swapgs();
+}
diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index f6efa21ec242..cf41901227ed 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -309,6 +309,8 @@ static __always_inline void __##func(struct pt_regs *regs)
 #ifdef CONFIG_X86_64
 __visible __entry_text
 void ist_paranoid_entry(unsigned long *cr3, unsigned long *gsbase);
+__visible __entry_text
+void ist_paranoid_exit(unsigned long cr3, unsigned long gsbase);
 
 /**
  * DECLARE_IDTENTRY_IST - Declare functions for IST handling IDT entry points
-- 
2.19.1.6.gb485710b