[PATCH V6 31/49] x86/entry: Skip CR3 write when the saved CR3 is kernel CR3 in RESTORE_CR3

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

When the original CR3 is kernel CR3, paranoid_entry() hasn't changed
the CR3, so the CR3 doesn't need to restored when paranoid_exit() in
the this case.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 arch/x86/entry/calling.h | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 996b041e92d2..9065c31d2875 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -231,14 +231,11 @@ For 32-bit we have the following conventions - kernel is built with
 .macro RESTORE_CR3 scratch_reg:req save_reg:req
 	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
 
-	ALTERNATIVE "jmp .Lwrcr3_\@", "", X86_FEATURE_PCID
-
-	/*
-	 * KERNEL pages can always resume with NOFLUSH as we do
-	 * explicit flushes.
-	 */
+	/* No need to restore when the saved CR3 is kernel CR3. */
 	bt	$PTI_USER_PGTABLE_BIT, \save_reg
-	jnc	.Lnoflush_\@
+	jnc	.Lend_\@
+
+	ALTERNATIVE "jmp .Lwrcr3_\@", "", X86_FEATURE_PCID
 
 	/*
 	 * Check if there's a pending flush for the user ASID we're
@@ -256,10 +253,6 @@ For 32-bit we have the following conventions - kernel is built with
 	SET_NOFLUSH_BIT \save_reg
 
 .Lwrcr3_\@:
-	/*
-	 * The CR3 write could be avoided when not changing its value,
-	 * but would require a CR3 read *and* a scratch register.
-	 */
 	movq	\save_reg, %cr3
 .Lend_\@:
 .endm
-- 
2.19.1.6.gb485710b