[PATCH v4 03/11] x86/startup_64: Simplify CR4 handling in startup code

[Ard Biesheuvel <ardb+git@google.com>]

From: Ard Biesheuvel <ardb@kernel.org>

When paging is enabled, the CR4.PAE and CR4.LA57 control bits cannot be
changed, and so they can simply be preserved rather than reason about
whether or not they need to be set. CR4.MCE should be preserved unless
the kernel was built without CONFIG_X86_MCE, in which case it must be
cleared.

CR4.PSE should be set explicitly, regardless of whether or not it was
set before.

CR4.PGE is set explicitly, and then cleared and set again after
programming CR3 in order to flush TLB entries based on global
translations. This makes the first assignment redundant, and can
therefore be omitted.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/x86/kernel/head_64.S | 24 ++++++--------------
 1 file changed, 7 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 3cac98c61066..7e76cc0b442a 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -185,6 +185,8 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	addq	$(init_top_pgt - __START_KERNEL_map), %rax
 1:
 
+	/* Create a mask of CR4 bits to preserve */
+	movl	$(X86_CR4_PAE | X86_CR4_LA57), %edx
 #ifdef CONFIG_X86_MCE
 	/*
 	 * Preserve CR4.MCE if the kernel will enable #MC support.
@@ -193,20 +195,11 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	 * configured will crash the system regardless of the CR4.MCE value set
 	 * here.
 	 */
-	movq	%cr4, %rcx
-	andl	$X86_CR4_MCE, %ecx
-#else
-	movl	$0, %ecx
-#endif
-
-	/* Enable PAE mode, PSE, PGE and LA57 */
-	orl	$(X86_CR4_PAE | X86_CR4_PSE | X86_CR4_PGE), %ecx
-#ifdef CONFIG_X86_5LEVEL
-	testb	$1, __pgtable_l5_enabled(%rip)
-	jz	1f
-	orl	$X86_CR4_LA57, %ecx
-1:
+	orl	$X86_CR4_MCE, %edx
 #endif
+	movq	%cr4, %rcx
+	andl	%edx, %ecx
+	btsl	$X86_CR4_PSE_BIT, %ecx
 	movq	%rcx, %cr4
 
 	/* Setup early boot stage 4-/5-level pagetables. */
@@ -226,11 +219,8 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	 * Do a global TLB flush after the CR3 switch to make sure the TLB
 	 * entries from the identity mapping are flushed.
 	 */
-	movq	%cr4, %rcx
-	movq	%rcx, %rax
-	xorq	$X86_CR4_PGE, %rcx
+	btsl	$X86_CR4_PGE_BIT, %ecx
 	movq	%rcx, %cr4
-	movq	%rax, %cr4
 
 	/* Ensure I am executing from virtual addresses */
 	movq	$1f, %rax
-- 
2.43.0.687.g38aa6559b0-goog