linux-kernel - [PATCH] x86/boot/64: Load kernel GDT in early boot

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Date: Thu, 22 Feb 2024 13:23:56 -0500
From: Brian Gerst <brgerst@...il.com>
To: linux-kernel@...r.kernel.org,
	x86@...nel.org
Cc: Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	"H . Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	Brian Gerst <brgerst@...il.com>
Subject: [PATCH] x86/boot/64: Load kernel GDT in early boot

Instead of loading a duplicate GDT just for early boot, load the kernel
GDT from its physical address.

Signed-off-by: Brian Gerst <brgerst@...il.com>
---
 arch/x86/include/asm/desc.h |  1 +
 arch/x86/kernel/head64.c    | 14 +++-----------
 2 files changed, 4 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index ec95fe44fa3a..62dc9f59ea76 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -46,6 +46,7 @@ struct gdt_page {
 } __attribute__((aligned(PAGE_SIZE)));
 
 DECLARE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page);
+DECLARE_INIT_PER_CPU(gdt_page);
 
 /* Provide the original GDT */
 static inline struct desc_struct *get_cpu_gdt_rw(unsigned int cpu)
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index dc0956067944..09c67e6add62 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -67,21 +67,12 @@ unsigned long vmemmap_base __ro_after_init = __VMEMMAP_BASE_L4;
 EXPORT_SYMBOL(vmemmap_base);
 #endif
 
-/*
- * GDT used on the boot CPU before switching to virtual addresses.
- */
-static struct desc_struct startup_gdt[GDT_ENTRIES] __initdata = {
-	[GDT_ENTRY_KERNEL32_CS]         = GDT_ENTRY_INIT(DESC_CODE32, 0, 0xfffff),
-	[GDT_ENTRY_KERNEL_CS]           = GDT_ENTRY_INIT(DESC_CODE64, 0, 0xfffff),
-	[GDT_ENTRY_KERNEL_DS]           = GDT_ENTRY_INIT(DESC_DATA64, 0, 0xfffff),
-};
-
 /*
  * Address needs to be set at runtime because it references the startup_gdt
  * while the kernel still uses a direct mapping.
  */
 static struct desc_ptr startup_gdt_descr __initdata = {
-	.size = sizeof(startup_gdt)-1,
+	.size = GDT_SIZE - 1,
 	.address = 0,
 };
 
@@ -624,7 +615,8 @@ void early_setup_idt(void)
 void __head startup_64_setup_env(unsigned long physbase)
 {
 	/* Load GDT */
-	startup_gdt_descr.address = (unsigned long)fixup_pointer(startup_gdt, physbase);
+	startup_gdt_descr.address =
+		(unsigned long)fixup_pointer(&init_per_cpu_var(gdt_page.gdt), physbase);
 	native_load_gdt(&startup_gdt_descr);
 
 	/* New GDT is live - reload data segment registers */

base-commit: c56ac217a3c00d12e23606e0635cc95d6eb8c36a
-- 
2.43.2