linux-kernel - [PATCH v2 4/7] x86/efi/mixed: Set up 1:1 mapping of lower 4GiB in the stub

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Message-ID: <20250210174941.3251435-13-ardb+git@google.com>
Date: Mon, 10 Feb 2025 18:49:46 +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 4/7] x86/efi/mixed: Set up 1:1 mapping of lower 4GiB in the stub

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

In preparation for dropping the dependency on startup_32 entirely in the
next patch, add the code that sets up the 1:1 mapping of the lower 4 GiB
of system RAM to the mixed mode stub.

The reload of CR3 after the long mode switch will be removed in a
subsequent patch, when it is no longer needed.

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

diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S
index 0b6b37b08f82..dca916c3e6f0 100644
--- a/arch/x86/boot/compressed/efi_mixed.S
+++ b/arch/x86/boot/compressed/efi_mixed.S
@@ -18,6 +18,7 @@
 #include <asm/asm-offsets.h>
 #include <asm/msr.h>
 #include <asm/page_types.h>
+#include <asm/pgtable_types.h>
 #include <asm/processor-flags.h>
 #include <asm/segment.h>
 #include <asm/setup.h>
@@ -52,6 +53,9 @@ SYM_FUNC_START(startup_64_mixed_mode)
 	mov	0(%rdx), %edi
 	mov	4(%rdx), %esi
 
+	leaq	(pte + 5 * PAGE_SIZE)(%rip), %rax
+	movq	%rax, %cr3		// reload after startup_32
+
 	/* Switch to the firmware's stack */
 	movl	efi32_boot_sp(%rip), %esp
 	andl	$~7, %esp
@@ -267,11 +271,32 @@ SYM_FUNC_START_LOCAL(efi32_entry)
 	movl	$_end - 1b, BP_init_size(%esi)
 	subl	$startup_32 - 1b, BP_init_size(%esi)
 
+	call	1f
+1:	pop	%edi
+
 	/* Disable paging */
 	movl	%cr0, %eax
 	btrl	$X86_CR0_PG_BIT, %eax
 	movl	%eax, %cr0
 
+	/* Set up 1:1 mapping */
+	leal	(pte - 1b)(%edi), %eax
+	movl	$_PAGE_PRESENT | _PAGE_RW | _PAGE_PSE, %ecx
+	leal	(_PAGE_PRESENT | _PAGE_RW)(%eax), %edx
+2:	movl	%ecx, (%eax)
+	addl	$8, %eax
+	addl	$PMD_SIZE, %ecx
+	jnc	2b
+
+	movl	$PAGE_SIZE, %ecx
+	.irpc	l, 0123
+	movl	%edx, \l * 8(%eax)
+	addl	%ecx, %edx
+	.endr
+	addl	%ecx, %eax
+	movl	%edx, (%eax)
+	movl	%eax, %cr3
+
 	jmp	startup_32
 SYM_FUNC_END(efi32_entry)
 
@@ -322,3 +347,7 @@ SYM_DATA_LOCAL(efi32_boot_ds, .word 0)
 SYM_DATA_LOCAL(efi32_boot_sp, .long 0)
 SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0, 0)
 SYM_DATA(efi_is64, .byte 1)
+
+	.bss
+	.balign PAGE_SIZE
+SYM_DATA_LOCAL(pte, .fill 6 * PAGE_SIZE, 1, 0)
-- 
2.48.1.362.g079036d154-goog