linux-kernel - Re: [PATCH v1 1/1] x86/fred: Fix init_task thread stack pointer initialization

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Date: Fri, 1 Mar 2024 20:17:44 -0800
From: Xin Li <xin@...or.com>
To: Brian Gerst <brgerst@...il.com>
Cc: linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com
Subject: Re: [PATCH v1 1/1] x86/fred: Fix init_task thread stack pointer
 initialization

On 3/1/2024 5:15 AM, Brian Gerst wrote:
> On Fri, Mar 1, 2024 at 3:41 AM Xin Li (Intel) <xin@...or.com> wrote:
> There is another spot in head_64.S that also needs this offset:

I checked all references to __end_init_task before sending out this
patch, and I doubt we need to make more similar changes.

First of all, "movq    TASK_threadsp(%rcx), %rsp" you added in 
3adee777ad0d ("x86/smpboot: Remove initial_stack on 64-bit") is exactly
what we need to set up %rsp for the init task.

> /* Set up the stack for verify_cpu() */
> leaq (__end_init_task - PTREGS_SIZE)(%rip), %rsp

As the comment says, it's a _temporary_ stack for calling verify_cpu()
(but only for BSP, as APs use a different bring up stack), at which
stage the concept of "task" has not formed. I'm thinking maybe it's
better to do:

/* Set up the stack for verify_cpu() */
leaq __end_init_task(%rip), %rsp

Previously it was "leaq    (__end_init_task - FRAME_SIZE)(%rip), %rsp",
but the kernel unwinder goes up only to secondary_startup_64_no_verify()
after the new way you introduced to set up %rsp for the init task, and
it seems to me that there is no point to subtract FRAME_SIZE or
PTREGS_SIZE.

On the other hand, TOP_OF_KERNEL_STACK_PADDING is required for x86_32,
but probably not for x86_64 (defined as 0 before FRED). The most
important usage of TOP_OF_KERNEL_STACK_PADDING is to get the pt_regs
pointer for a task, i.e., task_pt_regs(task), which assumes a fixed
offset from the top of a task stack, but also limits the space that
could be used by future hardware above the pt_regs structure. Thus I
prefer to limit the usage of TOP_OF_KERNEL_STACK_PADDING on x86_64.

Thanks!
     Xin