[PATCH V3 5/7] x86/entry: Add the C verion of SWITCH_TO_KERNEL_CR3 as switch_to_kernel_cr3()

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

Add the C version switch_to_kernel_cr3() which implements the macro
SWITCH_TO_KERNEL_CR3() in arch/x86/entry/calling.h.

No functional difference intended.

Note:
The compiler generates "AND $0xe7,%ah" (3 bytes) for the code
"cr3 = user_cr3 & ~PTI_USER_PGTABLE_AND_PCID_MASK" while the ASM code in
SWITCH_TO_KERNEL_CR3() results "AND $0xffffffffffffe7ff,%rax" (6 bytes).

The compiler generates lengthier code for "cr3 |= X86_CR3_PCID_NOFLUSH"
because it uses "MOVABS+OR" (13 bytes) rather than a single
"BTS" (5 bytes).

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
---
 arch/x86/entry/entry64.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/x86/entry/entry64.c b/arch/x86/entry/entry64.c
index ace73861c2a0..bd77cc8373ce 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -12,3 +12,27 @@
  * is the PTI user CR3 or both.
  */
 #include <asm/traps.h>
+
+#ifdef CONFIG_PAGE_TABLE_ISOLATION
+static __always_inline void pti_switch_to_kernel_cr3(unsigned long user_cr3)
+{
+	/*
+	 * Clear PCID and "PAGE_TABLE_ISOLATION bit", point CR3
+	 * at kernel pagetables:
+	 */
+	unsigned long cr3 = user_cr3 & ~PTI_USER_PGTABLE_AND_PCID_MASK;
+
+	if (static_cpu_has(X86_FEATURE_PCID))
+		cr3 |= X86_CR3_PCID_NOFLUSH;
+
+	native_write_cr3(cr3);
+}
+
+static __always_inline void switch_to_kernel_cr3(void)
+{
+	if (static_cpu_has(X86_FEATURE_PTI))
+		pti_switch_to_kernel_cr3(__native_read_cr3());
+}
+#else
+static __always_inline void switch_to_kernel_cr3(void) {}
+#endif
-- 
2.19.1.6.gb485710b