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

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Message-ID: <CALCETrWDYxZPFT=N6N3Ukci_xEU3OHS+0tajihuf+03j=0paoQ@mail.gmail.com>
Date:   Mon, 13 Nov 2017 18:25:24 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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>
Subject: Re: [RFC 6/7] x86/asm: Remap the TSS into the cpu entry area

On Mon, Nov 13, 2017 at 11:36 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Nov 13, 2017 at 11:22 AM, Dave Hansen <dave.hansen@...el.com> wrote:
>>
>> Aha, and here's the place that you need sizeof(tss_struct) to be nice
>> and page-aligned.
>
> No, it should _not_ be page-aligned. It should fit _within_ a page,
> but it 'struct tss_struct' now has something else in front of it, then
> page-aliging that is actually pointless.
>
> I forget what the actual size is, but aligning the hardware TSS struct
> to 128 bytes might be sufficient. It's not that big.

104 bytes, so it's probably already fine.  For anything except an
actual task switch, only the first 12 or so bytes matter.