linux-kernel - Re: [PATCH 07/16] x86/asm: Move SYSENTER_stack to the beginning of struct tss

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Date:   Tue, 21 Nov 2017 00:37:12 +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 07/16] x86/asm: Move SYSENTER_stack to the beginning of
 struct tss_struct

On Mon, 20 Nov 2017, Andy Lutomirski wrote:
>  struct tss_struct {
>  	/*
> -	 * The hardware state:
> +	 * Space for the temporary SYSENTER stack.  Used for the entry
> +	 * trampoline as well.  Size it such that tss_struct ends up
> +	 * as a multiple of PAGE_SIZE.  This calculation assumes that
> +	 * io_bitmap is a multiple of PAGE_SIZE (8192 bytes) plus one
> +	 * long.

I don't see how sizeof(tss_struct) is a multiple of PAGE_SIZE

canary	    	=    8
stack		=  512
hw_tss	    	=  104
io bitmap	= 8200
-------------------------
		  8824

The alignment is what blows it up to 3 * PAGE_SIZE

> +	 */
> +	unsigned long		SYSENTER_stack_canary;
> +	unsigned long		SYSENTER_stack[64];
> +
> +	/*
> +	 * The fixed hardware portion.  This must not cross a page boundary
> +	 * at risk of violating the SDM's advice and potentially triggering
> +	 * errata.
>  	 */
>  	struct x86_hw_tss	x86_tss;
>  
> @@ -338,15 +350,9 @@ struct tss_struct {
>  	 * be within the limit.
>  	 */
>  	unsigned long		io_bitmap[IO_BITMAP_LONGS + 1];
> +} __attribute__((__aligned__(PAGE_SIZE)));
>  

Thanks,

	tglx