[PATCH v2 16/20] x86/boot/64: Stop initializing TSS.sp0 at boot

[Andy Lutomirski]

In my quest to get rid of thread_struct::sp0, I want to clean up or
remove all of its readers.  Two of them are in cpu_init() (32-bit and
64-bit), and they aren't needed.  This is because we never enter
userspace at all on the threads that CPUs are initialized in.

Poison the initial TSS.sp0 and stop initializing it on CPU init.

The comment text mostly comes from Dave Hansen.  Thanks!

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/common.c | 12 ++++++++++--
 arch/x86/kernel/process.c    |  8 +++++++-
 2 files changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 079648bd85ed..adc02cb351e0 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1572,9 +1572,13 @@ void cpu_init(void)
 	initialize_tlbstate_and_flush();
 	enter_lazy_tlb(&init_mm, me);
 
-	load_sp0(current->thread.sp0);
+	/*
+	 * Initialize the TSS.  Don't bother initializing sp0, as the initial
+	 * task never enters user mode.
+	 */
 	set_tss_desc(cpu, t);
 	load_TR_desc();
+
 	load_mm_ldt(&init_mm);
 
 	clear_all_debug_regs();
@@ -1627,9 +1631,13 @@ void cpu_init(void)
 	initialize_tlbstate_and_flush();
 	enter_lazy_tlb(&init_mm, curr);
 
-	load_sp0(thread->sp0);
+	/*
+	 * Initialize the TSS.  Don't bother initializing sp0, as the initial
+	 * task never enters user mode.
+	 */
 	set_tss_desc(cpu, t);
 	load_TR_desc();
+
 	load_mm_ldt(&init_mm);
 
 	t->x86_tss.io_bitmap_base = offsetof(struct tss_struct, io_bitmap);
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index bd6b85fac666..ff8a9acbcf8b 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -48,7 +48,13 @@
  */
 __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
 	.x86_tss = {
-		.sp0 = TOP_OF_INIT_STACK,
+		/*
+		 * .sp0 is only used when entering ring 0 from a lower
+		 * privilege level.  Since the init task never runs anything
+		 * but ring 0 code, there is no need for a valid value here.
+		 * Poison it.
+		 */
+		.sp0 = (1UL << (BITS_PER_LONG-1)) + 1,
 #ifdef CONFIG_X86_32
 		.ss0 = __KERNEL_DS,
 		.ss1 = __KERNEL_CS,
-- 
2.13.6