linux-kernel - Re: [RFC 6/7] x86/asm: Remap the TSS into the cpu entry area

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Date:   Mon, 13 Nov 2017 11:22:44 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>
Cc:     Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 6/7] x86/asm: Remap the TSS into the cpu entry area

On 11/10/2017 08:05 PM, Andy Lutomirski wrote:
> diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
> index fbc9b7f4e35e..8a9ba5553cab 100644
> --- a/arch/x86/include/asm/fixmap.h
> +++ b/arch/x86/include/asm/fixmap.h
> @@ -52,6 +52,13 @@ extern unsigned long __FIXADDR_TOP;
>  struct cpu_entry_area
>  {
>  	char gdt[PAGE_SIZE];
> +
> +	/*
> +	 * The gdt is just below cpu_tss and thus serves (on x86_64) as a
> +	 * a read-only guard page for the SYSENTER stack at the bottom
> +	 * of the TSS region.
> +	 */
> +	struct tss_struct tss;
>  };
>  

Aha, and here's the place that you need sizeof(tss_struct) to be nice
and page-aligned.

But why don't we just do:

	char tss_space[PAGE_SIZE*something];

?