linux-kernel - Re: [PATCH 04/16] x86/fixmap: Generalize the GDT fixmap mechanism

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Date:   Mon, 20 Nov 2017 23:01:19 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     X86 ML <x86@...nel.org>, Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 04/16] x86/fixmap: Generalize the GDT fixmap mechanism

On Mon, 20 Nov 2017, Andy Lutomirski wrote:

Just a few nits.

>  /* Provide the fixmap address of the remapped GDT */
>  static inline struct desc_struct *get_cpu_gdt_ro(int cpu)
>  {
> -	unsigned int idx = get_cpu_gdt_ro_index(cpu);
> -	return (struct desc_struct *)__fix_to_virt(idx);
> +	return (struct desc_struct *)&get_cpu_entry_area(cpu)->gdt;
>  }
>  
>  /* Provide the current read-only GDT */
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index b0c505fe9a95..5ff8e251772a 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -44,6 +44,17 @@ extern unsigned long __FIXADDR_TOP;
>  			 PAGE_SIZE)
>  #endif
>  
> +/*
> + * cpu_entry_area is a percpu region in the fixmap that contains things
> + * need by the CPU and early entry/exit code.  Real types aren't used here

s/need/needed/

> + * to avoid circular header dependencies.

:(

> + */
> +struct cpu_entry_area
> +{
> +	char gdt[PAGE_SIZE];
> +};
> +
> +#define CPU_ENTRY_AREA_PAGES (sizeof(struct cpu_entry_area) / PAGE_SIZE)

> +static inline unsigned int __get_cpu_entry_area_page_index(int cpu, int page)
> +{
> +	BUILD_BUG_ON(sizeof(struct cpu_entry_area) % PAGE_SIZE != 0);
> +
> +	return FIX_CPU_ENTRY_AREA_BOTTOM - cpu*CPU_ENTRY_AREA_PAGES - page;
> +}
> +
> +#define __get_cpu_entry_area_offset_index(cpu, offset) ({		\
> +	BUILD_BUG_ON(offset % PAGE_SIZE != 0);				\
> +	__get_cpu_entry_area_page_index(cpu, offset / PAGE_SIZE);	\
> +	})
> +
> +#define get_cpu_entry_area_index(cpu, field)				\
> +	__get_cpu_entry_area_offset_index((cpu), offsetof(struct cpu_entry_area, field))

Any reason why those need to be macros?

> +static inline struct cpu_entry_area *get_cpu_entry_area(int cpu)
> +{
> +	return (struct cpu_entry_area *)
> +		__fix_to_virt(__get_cpu_entry_area_page_index(cpu, 0));
> +}
> +

Thanks,

	tglx