linux-kernel - Re: [PATCH 6/6] x86/vdso: Move out the CPU number store

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Date:   Tue, 5 Jun 2018 06:57:38 +0300
From:   Mika Penttilä <mika.penttila@...tfour.com>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Markus T Metzger <markus.t.metzger@...el.com>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] x86/vdso: Move out the CPU number store

On 06/04/2018 10:24 PM, Chang S. Bae wrote:
> The CPU (and node) number will be written, as early enough,
> to the segment limit of per CPU data and TSC_AUX MSR entry.
> The information has been retrieved by vgetcpu in user space
> and will be also loaded from the paranoid entry, when
> FSGSBASE enabled. So, it is moved out from vDSO to the CPU
> initialization path where IST setup is serialized.
> 
> Now, redundant setting of the segment in entry/vdso/vma.c
> was removed; a substantial code removal. It removes a
> hotplug notifier, makes a facility useful to both the kernel
> and userspace unconditionally available much sooner, and
> unification with i386. (Thanks to HPA for suggesting the
> cleanup)
> 
> Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Andi Kleen <ak@...ux.intel.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> ---

> diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
> index ea554f8..e716e94 100644
> --- a/arch/x86/kernel/setup_percpu.c
> +++ b/arch/x86/kernel/setup_percpu.c
> @@ -155,12 +155,21 @@ static void __init pcpup_populate_pte(unsigned long addr)
>  
>  static inline void setup_percpu_segment(int cpu)
>  {
> -#ifdef CONFIG_X86_32
> -	struct desc_struct d = GDT_ENTRY_INIT(0x8092, per_cpu_offset(cpu),
> -					      0xFFFFF);
> +#ifdef CONFIG_NUMA
> +	unsigned long node = early_cpu_to_node(cpu);
> +#else
> +	unsigned long node = 0;
> +#endif
> +	struct desc_struct d = GDT_ENTRY_INIT(0x0, per_cpu_offset(cpu),
> +			   make_lsl_tscp(cpu, node));
> +
> +	d.type = 5;	/* R0 data, expand down, accessed */
> +	d.dpl = 3;	/* Visible to user code */
> +	d.s = 1;	/* Not a system segment */
> +	d.p = 1;	/* Present */
> +	d.d = 1;	/* 32-bit */
>  
>  	write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_PERCPU, &d, DESCTYPE_S);
> -#endif
>  }


This won't work on X86-32 because it actually uses the segment limit with fs: access. So there 
is a reason why the lsl based method is X86-64 only.


-Mika

>  
>  void __init setup_per_cpu_areas(void)
>