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Subject: Re: [RFC 6/7] x86/asm: Remap the TSS into the cpu entry area
To: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>
References: <cover.1510371795.git.luto@kernel.org>
 <5b424fefa9230917995d0584b40bc539a9bd9224.1510371795.git.luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <3e2bdf66-cbd4-4e78-ead1-e5c99d2a6d08@intel.com>
Date: Mon, 13 Nov 2017 11:22:44 -0800
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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];

?