[PATCH 03/39] x86/entry/32: Load task stack from x86_tss.sp1 in SYSENTER handler

[Joerg Roedel]

From: Joerg Roedel <jroedel@xxxxxxx>

We want x86_tss.sp0 point to the entry stack later to use
it as a trampoline stack for other kernel entry points
besides SYSENTER.

So store the task stack pointer in x86_tss.sp1, which is
otherwise unused by the hardware, as Linux doesn't make use
of Ring 1.

Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
---
 arch/x86/kernel/asm-offsets_32.c | 9 +++++++--
 arch/x86/kernel/process_32.c     | 2 ++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/asm-offsets_32.c b/arch/x86/kernel/asm-offsets_32.c
index 15b3f45..82826f2 100644
--- a/arch/x86/kernel/asm-offsets_32.c
+++ b/arch/x86/kernel/asm-offsets_32.c
@@ -46,9 +46,14 @@ void foo(void)
 	OFFSET(saved_context_gdt_desc, saved_context, gdt_desc);
 	BLANK();
 
-	/* Offset from the entry stack to task stack stored in TSS */
+	/*
+	 * Offset from the entry stack to task stack stored in TSS. Kernel entry
+	 * happens on the per-cpu entry-stack, and the asm code switches to the
+	 * task-stack pointer stored in x86_tss.sp1, which is a copy of
+	 * task->thread.sp0 where entry code can find it.
+	 */
 	DEFINE(TSS_entry2task_stack,
-	       offsetof(struct cpu_entry_area, tss.x86_tss.sp0) -
+	       offsetof(struct cpu_entry_area, tss.x86_tss.sp1) -
 	       offsetofend(struct cpu_entry_area, entry_stack_page.stack));
 
 #ifdef CONFIG_STACKPROTECTOR
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 0ae659d..ec62cc7 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -290,6 +290,8 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
 	this_cpu_write(cpu_current_top_of_stack,
 		       (unsigned long)task_stack_page(next_p) +
 		       THREAD_SIZE);
+	/* SYSENTER reads the task-stack from tss.sp1 */
+	this_cpu_write(cpu_tss_rw.x86_tss.sp1, next_p->thread.sp0);
 
 	/*
 	 * Restore %gs if needed (which is common)
-- 
2.7.4