[PATCH v3 05/19] x86/startup_64: Simplify CR4 handling in startup code

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

When executing in long mode, the CR4.PAE and CR4.LA57 control bits
cannot be updated, and so they can simply be preserved rather than
reason about whether or not they need to be set. CR4.PSE has no effect
in long mode so it can be omitted.

CR4.PGE is used to flush the TLBs, by clearing it if it was set, and
subsequently re-enabling it. So there is no need to set it just to
disable and re-enable it later.

CR4.MCE must be preserved unless the kernel was built without
CONFIG_X86_MCE, in which case it must be cleared.

Reimplement the above logic in a more straight-forward way, by defining
a mask of CR4 bits to preserve, and applying that to CR4 at the point
where it needs to be updated anyway.

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 arch/x86/kernel/head_64.S | 27 ++++++++------------
 1 file changed, 10 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 6d24c2014759..ca46995205d4 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -179,6 +179,12 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 
 1:
 
+	/*
+	 * Define a mask of CR4 bits to preserve. PAE and LA57 cannot be
+	 * modified while paging remains enabled. PGE will be toggled below if
+	 * it is already set.
+	 */
+	movl	$(X86_CR4_PAE | X86_CR4_PGE | X86_CR4_LA57), %edx
 #ifdef CONFIG_X86_MCE
 	/*
 	 * Preserve CR4.MCE if the kernel will enable #MC support.
@@ -187,22 +193,9 @@ 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
+	orl	$X86_CR4_MCE, %edx
 #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:
-#endif
-	movq	%rcx, %cr4
-
 	/*
 	 * Switch to new page-table
 	 *
@@ -218,10 +211,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	 * entries from the identity mapping are flushed.
 	 */
 	movq	%cr4, %rcx
-	movq	%rcx, %rax
-	xorq	$X86_CR4_PGE, %rcx
+	andl	%edx, %ecx
+0:	btcl	$X86_CR4_PGE_BIT, %ecx
 	movq	%rcx, %cr4
-	movq	%rax, %cr4
+	jc	0b
 
 	/* Ensure I am executing from virtual addresses */
 	movq	$1f, %rax
-- 
2.43.0.429.g432eaa2c6b-goog