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Date: Fri, 22 Mar 2024 04:47:51 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
To: Brian Gerst <brgerst@gmail.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org
CC: Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>, Xin Li <xin@zytor.com>
Subject: Re: [PATCH] x86/boot: Simplify boot stack setup
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On March 21, 2024 11:05:06 AM PDT, Brian Gerst <brgerst@gmail=2Ecom> wrote:
>Define the symbol __top_init_kernel_stack instead of duplicating
>the offset from __end_init_task in multiple places=2E
>
>Signed-off-by: Brian Gerst <brgerst@gmail=2Ecom>
>---
> arch/x86/include/asm/processor=2Eh |  6 ++----
> arch/x86/kernel/head_32=2ES        | 11 +----------
> arch/x86/kernel/head_64=2ES        |  2 +-
> arch/x86/kernel/vmlinux=2Elds=2ES    |  3 +++
> arch/x86/xen/xen-head=2ES          |  2 +-
> 5 files changed, 8 insertions(+), 16 deletions(-)
>
>diff --git a/arch/x86/include/asm/processor=2Eh b/arch/x86/include/asm/pr=
ocessor=2Eh
>index 438c0c8f596a=2E=2E89ed5237e79f 100644
>--- a/arch/x86/include/asm/processor=2Eh
>+++ b/arch/x86/include/asm/processor=2Eh
>@@ -636,12 +636,10 @@ static __always_inline void prefetchw(const void *x=
)
> #define KSTK_ESP(task)		(task_pt_regs(task)->sp)
>=20
> #else
>-extern unsigned long __end_init_task[];
>+extern unsigned long __top_init_kernel_stack[];
>=20
> #define INIT_THREAD {							\
>-	=2Esp	=3D (unsigned long)&__end_init_task -			\
>-		  TOP_OF_KERNEL_STACK_PADDING -				\
>-		  sizeof(struct pt_regs),				\
>+	=2Esp	=3D (unsigned long)&__top_init_kernel_stack,		\
> }
>=20
> extern unsigned long KSTK_ESP(struct task_struct *task);
>diff --git a/arch/x86/kernel/head_32=2ES b/arch/x86/kernel/head_32=2ES
>index b50f3641c4d6=2E=2Ea9de527ba5c4 100644
>--- a/arch/x86/kernel/head_32=2ES
>+++ b/arch/x86/kernel/head_32=2ES
>@@ -44,9 +44,6 @@
> #define X86_CAPABILITY	new_cpu_data+CPUINFO_x86_capability
> #define X86_VENDOR_ID	new_cpu_data+CPUINFO_x86_vendor_id
>=20
>-
>-#define SIZEOF_PTREGS 17*4
>-
> /*
>  * Worst-case size of the kernel mapping we need to make:
>  * a relocatable kernel can live anywhere in lowmem, so we need to be ab=
le
>@@ -488,13 +485,7 @@ SYM_DATA_END(initial_page_table)
>=20
> =2Edata
> =2Ebalign 4
>-/*
>- * The SIZEOF_PTREGS gap is a convention which helps the in-kernel unwin=
der
>- * reliably detect the end of the stack=2E
>- */
>-SYM_DATA(initial_stack,
>-		=2Elong init_thread_union + THREAD_SIZE -
>-		SIZEOF_PTREGS - TOP_OF_KERNEL_STACK_PADDING)
>+SYM_DATA(initial_stack, =2Elong __top_init_kernel_stack)
>=20
> __INITRODATA
> int_msg:
>diff --git a/arch/x86/kernel/head_64=2ES b/arch/x86/kernel/head_64=2ES
>index d8198fbd70e5=2E=2Eb11526869a40 100644
>--- a/arch/x86/kernel/head_64=2ES
>+++ b/arch/x86/kernel/head_64=2ES
>@@ -66,7 +66,7 @@ SYM_CODE_START_NOALIGN(startup_64)
> 	mov	%rsi, %r15
>=20
> 	/* Set up the stack for verify_cpu() */
>-	leaq	(__end_init_task - TOP_OF_KERNEL_STACK_PADDING - PTREGS_SIZE)(%rip=
), %rsp
>+	leaq	__top_init_kernel_stack(%rip), %rsp
>=20
> 	/* Setup GSBASE to allow stack canary access for C code */
> 	movl	$MSR_GS_BASE, %ecx
>diff --git a/arch/x86/kernel/vmlinux=2Elds=2ES b/arch/x86/kernel/vmlinux=
=2Elds=2ES
>index a20409b0a3f2=2E=2Ed430880175f2 100644
>--- a/arch/x86/kernel/vmlinux=2Elds=2ES
>+++ b/arch/x86/kernel/vmlinux=2Elds=2ES
>@@ -167,6 +167,9 @@ SECTIONS
> 		/* init_task */
> 		INIT_TASK_DATA(THREAD_SIZE)
>=20
>+		/* equivalent to task_pt_regs(&init_task) */
>+		__top_init_kernel_stack =3D __end_init_task - TOP_OF_KERNEL_STACK_PADD=
ING - PTREGS_SIZE;
>+
> #ifdef CONFIG_X86_32
> 		/* 32 bit has nosave before _edata */
> 		NOSAVE_DATA
>diff --git a/arch/x86/xen/xen-head=2ES b/arch/x86/xen/xen-head=2ES
>index 04101b984f24=2E=2E758bcd47b72d 100644
>--- a/arch/x86/xen/xen-head=2ES
>+++ b/arch/x86/xen/xen-head=2ES
>@@ -49,7 +49,7 @@ SYM_CODE_START(startup_xen)
> 	ANNOTATE_NOENDBR
> 	cld
>=20
>-	leaq	(__end_init_task - TOP_OF_KERNEL_STACK_PADDING - PTREGS_SIZE)(%rip=
), %rsp
>+	leaq	__top_init_kernel_stack(%rip), %rsp
>=20
> 	/* Set up %gs=2E
> 	 *
>
>base-commit: e1826833c3a9abb9d1fe4b938eca0e25eeafb39f

Acked-by: H=2E Peter Anvin (Intel) <hpa@zytor=2Ecom>