linux-kernel - [PATCH v1 3/3] x86: Get rid of TOP_OF_KERNEL_STACK_PADDING on x86

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Date: Mon, 17 Jun 2024 01:45:15 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org
Cc: luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        peterz@...radead.org, brgerst@...il.com
Subject: [PATCH v1 3/3] x86: Get rid of TOP_OF_KERNEL_STACK_PADDING on x86_64

Because task_pt_regs() is now just an alias of thread_info.user_pt_regs,
and no matter whether FRED is enabled or not a user level event frame on
x86_64 is always pushed from top of current task kernel stack, i.e.,
'(unsigned long)task_stack_page(task) + THREAD_SIZE', there is no meaning
to keep TOP_OF_KERNEL_STACK_PADDING on x86_64, thus remove it.

Signed-off-by: Xin Li (Intel) <xin@...or.com>
---
 arch/x86/include/asm/processor.h   |  6 ++++--
 arch/x86/include/asm/switch_to.h   |  2 +-
 arch/x86/include/asm/thread_info.h | 10 ----------
 arch/x86/kernel/process.c          |  3 +--
 4 files changed, 6 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 91803844c4d7..9c5294f6d923 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -644,8 +644,6 @@ extern unsigned long __end_init_stack[];
  */
 #define TOP_OF_INIT_STACK ((unsigned long)&__end_init_stack)
 
-#define task_top_of_stack(task) ((unsigned long)(task_pt_regs(task) + 1))
-
 /*
  * task_pt_regs() no longer converts a fixed offset from top of a task
  * kernel stack to a pt_regs structure pointer, but rather returns
@@ -660,6 +658,9 @@ extern unsigned long __end_init_stack[];
 #define task_pt_regs(task) ((task)->thread_info.user_pt_regs)
 
 #ifdef CONFIG_X86_32
+#define task_top_of_stack(task) ((unsigned long)task_stack_page(task) + THREAD_SIZE	\
+				 - TOP_OF_KERNEL_STACK_PADDING)
+
 #define INIT_THREAD  {							  \
 	.sp0			= TOP_OF_INIT_STACK,			  \
 	.sysenter_cs		= __KERNEL_CS,				  \
@@ -669,6 +670,7 @@ extern unsigned long __end_init_stack[];
 
 #else
 extern unsigned long __top_init_kernel_stack[];
+#define task_top_of_stack(task) ((unsigned long)task_stack_page(task) + THREAD_SIZE)
 
 #define INIT_THREAD {							\
 	.sp	= (unsigned long)&__top_init_kernel_stack,		\
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index c3bd0c0758c9..902f1612ef3f 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -72,7 +72,7 @@ static inline void update_task_stack(struct task_struct *task)
 #else
 	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
 		/* WRMSRNS is a baseline feature for FRED. */
-		wrmsrns(MSR_IA32_FRED_RSP0, (unsigned long)task_stack_page(task) + THREAD_SIZE);
+		wrmsrns(MSR_IA32_FRED_RSP0, task_top_of_stack(task));
 	} else if (cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		/* Xen PV enters the kernel on the thread stack. */
 		load_sp0(task_top_of_stack(task));
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index 326268d440cf..331a6f32a0be 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -30,10 +30,6 @@
  *
  * In vm86 mode, the hardware frame is much longer still, so add 16
  * bytes to make room for the real-mode segments.
- *
- * x86-64 has a fixed-length stack frame, but it depends on whether
- * or not FRED is enabled. Future versions of FRED might make this
- * dynamic, but for now it is always 2 words longer.
  */
 #ifdef CONFIG_X86_32
 # ifdef CONFIG_VM86
@@ -41,12 +37,6 @@
 # else
 #  define TOP_OF_KERNEL_STACK_PADDING 8
 # endif
-#else /* x86-64 */
-# ifdef CONFIG_X86_FRED
-#  define TOP_OF_KERNEL_STACK_PADDING (2 * 8)
-# else
-#  define TOP_OF_KERNEL_STACK_PADDING 0
-# endif
 #endif
 
 /*
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 787a402e4ead..99f9887f710e 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -116,9 +116,8 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
  */
 void arch_init_user_pt_regs(struct task_struct *tsk)
 {
-	unsigned long top_of_stack = (unsigned long)task_stack_page(tsk) + THREAD_SIZE;
+	unsigned long top_of_stack = task_top_of_stack(tsk);
 
-	top_of_stack -= TOP_OF_KERNEL_STACK_PADDING;
 	tsk->thread_info.user_pt_regs = (struct pt_regs *)top_of_stack - 1;
 }
 
-- 
2.45.1