[PATCH v5 29/75] x86/head/64: Install startup GDT

Joerg Roedel
2020-07-24T16:09:15Z

Handling exceptions during boot requires a working GDT. The kernel GDT
can't be used on the direct mapping, so load a startup GDT and setup
segments.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
---
arch/x86/include/asm/setup.h | 1 +
arch/x86/kernel/head64.c | 33 +++++++++++++++++++++++++++++++++
arch/x86/kernel/head_64.S | 14 ++++++++++++++
3 files changed, 48 insertions(+)

diff --git a/arch/x86/include/asm/setup.h b/arch/x86/include/asm/setup.h
index 84b645cc8bc9..5c2fd05bd52c 100644
--- a/arch/x86/include/asm/setup.h
+++ b/arch/x86/include/asm/setup.h
@@ -48,6 +48,7 @@ extern void reserve_standard_io_resources(void);
extern void i386_reserve_resources(void);
extern unsigned long __startup_64(unsigned long physaddr, struct boot_params *bp);
extern unsigned long __startup_secondary_64(void);
+extern void startup_64_setup_env(unsigned long physbase);
extern int early_make_pgtable(unsigned long address);

#ifdef CONFIG_X86_INTEL_MID
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index cbb71c1b574f..8c82be44be94 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -61,6 +61,24 @@ 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] = {
+ [GDT_ENTRY_KERNEL32_CS] = GDT_ENTRY_INIT(0xc09b, 0, 0xfffff),
+ [GDT_ENTRY_KERNEL_CS] = GDT_ENTRY_INIT(0xa09b, 0, 0xfffff),
+ [GDT_ENTRY_KERNEL_DS] = GDT_ENTRY_INIT(0xc093, 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 = {
+ .size = sizeof(startup_gdt),
+ .address = 0,
+};
+
#define __head __section(.head.text)

static void __head *fixup_pointer(void *ptr, unsigned long physaddr)
@@ -489,3 +507,18 @@ void __init x86_64_start_reservations(char *real_mode_data)

start_kernel();
}
+
+/*
+ * Setup boot CPU state needed before kernel switches to virtual addresses.
+ */
+void __head startup_64_setup_env(unsigned long physbase)
+{
+ /* Load GDT */
+ startup_gdt_descr.address = (unsigned long)fixup_pointer(startup_gdt, physbase);
+ native_load_gdt(&startup_gdt_descr);
+
+ /* New GDT is live - reload data segment registers */
+ asm volatile("movl %%eax, %%ds\n"
+ "movl %%eax, %%ss\n"
+ "movl %%eax, %%es\n" : : "a"(__KERNEL_DS) : "memory");
+}
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 16da4ac01597..2b2e91627221 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -73,6 +73,20 @@ SYM_CODE_START_NOALIGN(startup_64)
/* Set up the stack for verify_cpu(), similar to initial_stack below */
leaq (__end_init_task - SIZEOF_PTREGS)(%rip), %rsp

+ leaq _text(%rip), %rdi
+ pushq %rsi
+ call startup_64_setup_env
+ popq %rsi
+
+ /* Now switch to __KERNEL_CS so IRET works reliably */
+ pushq $__KERNEL_CS
+ leaq .Lon_kernel_cs(%rip), %rax
+ pushq %rax
+ lretq
+
+.Lon_kernel_cs:
+ UNWIND_HINT_EMPTY
+
/* Sanitize CPU configuration */
call verify_cpu

--
2.27.0