linux-kernel - [PATCH v2 3/7] x86/efi/mixed: Factor out and clean up long mode entry

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Message-ID: <20250210174941.3251435-12-ardb+git@google.com>
Date: Mon, 10 Feb 2025 18:49:45 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-efi@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, hdegoede@...hat.com, 
	Ard Biesheuvel <ardb@...nel.org>
Subject: [PATCH v2 3/7] x86/efi/mixed: Factor out and clean up long mode entry

From: Ard Biesheuvel <ardb@...nel.org>

Entering long mode involves setting the EFER_LME and CR4.PAE bits before
enabling paging by setting CR0.PG bit.

It also involves disabling interrupts, given that the firmware's 32-bit
IDT becomes invalid as soon as the CPU transitions into long mode.

Reloading the CR3 register is not necessary at boot time, given that the
EFI firmware as well as the kernel's EFI stub use a 1:1 mapping of the
32-bit addressable memory in the system.

Break out this code into a separate helper for clarity, and so that it
can be reused in a subsequent patch.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/boot/compressed/efi_mixed.S | 29 ++++++++++----------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S
index b7886e2591fc..0b6b37b08f82 100644
--- a/arch/x86/boot/compressed/efi_mixed.S
+++ b/arch/x86/boot/compressed/efi_mixed.S
@@ -170,10 +170,6 @@ SYM_FUNC_START_LOCAL(efi_enter32)
 	movl	%edx, %gs
 	movl	%edx, %ss
 
-	/* Reload pgtables */
-	movl	%cr3, %eax
-	movl	%eax, %cr3
-
 	/* Disable paging */
 	movl	%cr0, %eax
 	btrl	$X86_CR0_PG_BIT, %eax
@@ -199,30 +195,35 @@ SYM_FUNC_START_LOCAL(efi_enter32)
 	lidtl	16(%ebx)
 	lgdtl	(%ebx)
 
+	xorl	%eax, %eax
+	lldt	%ax
+
+	call	efi32_enable_long_mode
+
+	pushl	$__KERNEL_CS
+	pushl	%ebp
+	lret
+SYM_FUNC_END(efi_enter32)
+
+SYM_FUNC_START_LOCAL(efi32_enable_long_mode)
 	movl	%cr4, %eax
 	btsl	$(X86_CR4_PAE_BIT), %eax
 	movl	%eax, %cr4
 
-	movl	%cr3, %eax
-	movl	%eax, %cr3
-
 	movl	$MSR_EFER, %ecx
 	rdmsr
 	btsl	$_EFER_LME, %eax
 	wrmsr
 
-	xorl	%eax, %eax
-	lldt	%ax
-
-	pushl	$__KERNEL_CS
-	pushl	%ebp
+	/* Disable interrupts - the firmware's IDT does not work in long mode */
+	cli
 
 	/* Enable paging */
 	movl	%cr0, %eax
 	btsl	$X86_CR0_PG_BIT, %eax
 	movl	%eax, %cr0
-	lret
-SYM_FUNC_END(efi_enter32)
+	ret
+SYM_FUNC_END(efi32_enable_long_mode)
 
 /*
  * This is the common EFI stub entry point for mixed mode.
-- 
2.48.1.362.g079036d154-goog