[PATCH 18/24] x86/entry: Add the C version ist_switch_to_kernel_cr3()

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

It switches the CR3 to kernel CR3 and returns the original CR3, and
the caller should save the return value.

It is the C version of SAVE_AND_SWITCH_TO_KERNEL_CR3.

Not functional difference intended.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 arch/x86/entry/traps.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/entry/traps.c b/arch/x86/entry/traps.c
index da55565d43ef..9d6753d0b9fc 100644
--- a/arch/x86/entry/traps.c
+++ b/arch/x86/entry/traps.c
@@ -785,8 +785,23 @@ static __always_inline void switch_to_kernel_cr3(void)
 	if (static_cpu_has(X86_FEATURE_PTI))
 		pti_switch_to_kernel_cr3(__native_read_cr3());
 }
+
+static __always_inline unsigned long ist_switch_to_kernel_cr3(void)
+{
+	unsigned long cr3 = 0;
+
+	if (static_cpu_has(X86_FEATURE_PTI)) {
+		cr3 = __native_read_cr3();
+
+		if (cr3 & PTI_USER_PGTABLE_MASK)
+			pti_switch_to_kernel_cr3(cr3);
+	}
+
+	return cr3;
+}
 #else
 static __always_inline void switch_to_kernel_cr3(void) {}
+static __always_inline unsigned long ist_switch_to_kernel_cr3(void) { return 0; }
 #endif
 
 /*
-- 
2.19.1.6.gb485710b