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Date: Tue, 16 Jan 2018 22:03:19 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Joerg Roedel <joro@8bytes.org>
cc: Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [PATCH 09/16] x86/mm/pti: Clone CPU_ENTRY_AREA on PMD level on
 x86_32
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On Tue, 16 Jan 2018, Joerg Roedel wrote:
> +#ifdef CONFIG_X86_64
>  /*
>   * Clone a single p4d (i.e. a top-level entry on 4-level systems and a
>   * next-level entry on 5-level systems.
> @@ -322,13 +323,29 @@ static void __init pti_clone_p4d(unsigned long addr)
>  	kernel_p4d = p4d_offset(kernel_pgd, addr);
>  	*user_p4d = *kernel_p4d;
>  }
> +#endif
>  
>  /*
>   * Clone the CPU_ENTRY_AREA into the user space visible page table.
>   */
>  static void __init pti_clone_user_shared(void)
>  {
> +#ifdef CONFIG_X86_32
> +	/*
> +	 * On 32 bit PAE systems with 1GB of Kernel address space there is only
> +	 * one pgd/p4d for the whole kernel. Cloning that would map the whole
> +	 * address space into the user page-tables, making PTI useless. So clone
> +	 * the page-table on the PMD level to prevent that.
> +	 */
> +	unsigned long start, end;
> +
> +	start = CPU_ENTRY_AREA_BASE;
> +	end   = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES);
> +
> +	pti_clone_pmds(start, end, _PAGE_GLOBAL);
> +#else
>  	pti_clone_p4d(CPU_ENTRY_AREA_BASE);
> +#endif
>  }

Just a minor nit. You already wrap pti_clone_p4d() into X86_64. So it would
be cleaner to do:

  	kernel_p4d = p4d_offset(kernel_pgd, addr);
  	*user_p4d = *kernel_p4d;
}

static void __init pti_clone_user_shared(void)
{
  	pti_clone_p4d(CPU_ENTRY_AREA_BASE);
}

#else /* CONFIG_X86_64 */

/*
 * Big fat comment.
 */
static void __init pti_clone_user_shared(void)
{
	....
}
#endif /* !CONFIG_X86_64 */

Thanks,

	tglx