linux-kernel - Re: [PATCH 10/24] x86/mm/kaiser: Map the entry stack variables

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Date:   Mon, 27 Nov 2017 18:29:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 10/24] x86/mm/kaiser: Map the entry stack variables

On Mon, Nov 27, 2017 at 11:49:09AM +0100, Ingo Molnar wrote:
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 7b348cc33e2d..f4f4ab8525bd 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1515,7 +1515,7 @@ EXPORT_PER_CPU_SYMBOL(__preempt_count);
>   * the top of the kernel stack.  Use an extra percpu variable to track the
>   * top of the kernel stack directly.
>   */
> -DEFINE_PER_CPU(unsigned long, cpu_current_top_of_stack) =
> +DEFINE_PER_CPU_USER_MAPPED(unsigned long, cpu_current_top_of_stack) =
>  	(unsigned long)&init_thread_union + THREAD_SIZE;
>  EXPORT_PER_CPU_SYMBOL(cpu_current_top_of_stack);

I can't find where we would access this one without already having
switched to kernel CR3.

A kernel with this entire patch reverted seems to boot for me.