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

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Message-ID: <CA+55aFzVWuVzzsTnhQu5H8kkUjVvsGuiqqqsMx6Pp0sqK=Mcsw@mail.gmail.com>
Date:   Mon, 13 Nov 2017 11:36:22 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dave Hansen <dave.hansen@...el.com>
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>
Subject: Re: [RFC 6/7] x86/asm: Remap the TSS into the cpu entry area

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.

Of course, we've had issues with "big" alignments before, in that they
haven't been reliable because the base isn't reliably aligned (the
stack being the worst case, but even standard data sections have had
issues). It's partly why we have special page-aligned sections.

                Linus