[PATCH V2 21/41] x86/entry: Add the C version ist_restore_cr3()

[Lai Jiangshan <jiangshanlai@gmail.com>]

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

It implements the C version of RESTORE_CR3().

Not functional difference intended except the ASM code uses bit test
and clear operations while the C version uses mask check and 'AND'
operations.  The resulted asm code of both versions are very similar.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/entry/entry64.c | 46 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/arch/x86/entry/entry64.c b/arch/x86/entry/entry64.c
index faee44a3d1d8..2db9ae3508f1 100644
--- a/arch/x86/entry/entry64.c
+++ b/arch/x86/entry/entry64.c
@@ -8,6 +8,7 @@
  * environments that the GS base is user controlled value, or the CR3
  * is PTI user CR3 or both.
  */
+#include <asm/tlbflush.h>
 #include <asm/traps.h>
 
 extern unsigned char asm_load_gs_index_gs_change[];
@@ -27,6 +28,26 @@ static __always_inline void pti_switch_to_kernel_cr3(unsigned long user_cr3)
 	native_write_cr3(cr3);
 }
 
+static __always_inline void pti_switch_to_user_cr3(unsigned long user_cr3)
+{
+#define KERN_PCID_MASK (CR3_PCID_MASK & ~PTI_USER_PCID_MASK)
+
+	if (static_cpu_has(X86_FEATURE_PCID)) {
+		int pcid = user_cr3 & KERN_PCID_MASK;
+		unsigned short pcid_mask = 1ull << pcid;
+
+		/*
+		 * Check if there's a pending flush for the user ASID we're
+		 * about to set.
+		 */
+		if (!(this_cpu_read(cpu_tlbstate.user_pcid_flush_mask) & pcid_mask))
+			user_cr3 |= X86_CR3_PCID_NOFLUSH;
+		else
+			this_cpu_and(cpu_tlbstate.user_pcid_flush_mask, ~pcid_mask);
+	}
+	native_write_cr3(user_cr3);
+}
+
 static __always_inline void switch_to_kernel_cr3(void)
 {
 	if (static_cpu_has(X86_FEATURE_PTI))
@@ -46,9 +67,34 @@ static __always_inline unsigned long ist_switch_to_kernel_cr3(void)
 
 	return cr3;
 }
+
+static __always_inline void ist_restore_cr3(unsigned long cr3)
+{
+	if (!static_cpu_has(X86_FEATURE_PTI))
+		return;
+
+	if (unlikely(cr3 & PTI_USER_PGTABLE_MASK)) {
+		pti_switch_to_user_cr3(cr3);
+		return;
+	}
+
+	/*
+	 * KERNEL pages can always resume with NOFLUSH as we do
+	 * explicit flushes.
+	 */
+	if (static_cpu_has(X86_FEATURE_PCID))
+		cr3 |= X86_CR3_PCID_NOFLUSH;
+
+	/*
+	 * The CR3 write could be avoided when not changing its value,
+	 * but would require a CR3 read.
+	 */
+	native_write_cr3(cr3);
+}
 #else
 static __always_inline void switch_to_kernel_cr3(void) {}
 static __always_inline unsigned long ist_switch_to_kernel_cr3(void) { return 0; }
+static __always_inline void ist_restore_cr3(unsigned long cr3) {}
 #endif
 
 /*
-- 
2.19.1.6.gb485710b