linux-kernel - Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

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Date:   Mon, 20 Nov 2017 21:22:53 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        luto@...nel.org, torvalds@...ux-foundation.org,
        keescook@...gle.com, hughd@...gle.com, x86@...nel.org
Subject: Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

On Fri, 10 Nov 2017, Dave Hansen wrote:
>  	__set_fixmap(get_cpu_gdt_ro_index(cpu), get_cpu_gdt_paddr(cpu), prot);
> +
> +	/* CPU 0's mapping is done in kaiser_init() */
> +	if (cpu) {
> +		int ret;
> +
> +		ret = kaiser_add_mapping((unsigned long) get_cpu_gdt_ro(cpu),
> +					 PAGE_SIZE, __PAGE_KERNEL_RO);
> +		/*
> +		 * We do not have a good way to fail CPU bringup.
> +		 * Just WARN about it and hope we boot far enough
> +		 * to get a good log out.
> +		 */

The GDT fixmap can be set up before the CPU is started. There is no reason
to do that in cpu_init().

> +
> +	/*
> +	 * We could theoretically do this in setup_fixmap_gdt().
> +	 * But, we would need to rewrite the above page table
> +	 * allocation code to use the bootmem allocator.  The
> +	 * buddy allocator is not available at the time that we
> +	 * call setup_fixmap_gdt() for CPU 0.
> +	 */
> +	kaiser_add_user_map_early(get_cpu_gdt_ro(0), PAGE_SIZE,
> +				  __PAGE_KERNEL_RO | _PAGE_GLOBAL);

This one is needs to stay.

Thanks,

	tglx