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

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Date:   Mon, 20 Nov 2017 17:25:33 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>, 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, Nov 20, 2017 at 3:37 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 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

Whoops!  That *was* correct in the RFC version version, but I changed
it and failed to fix the comment.