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From:   Brian Gerst <brgerst@gmail.com>
Date:   Wed, 6 Jun 2018 15:07:15 -0400
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Subject: Re: [PATCH v2 7/8] x86/segments/32: Introduce CPU_NUMBER segment
To:     Andy Lutomirski <luto@kernel.org>
Cc:     "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Jun 6, 2018 at 1:16 PM, Andy Lutomirski <luto@kernel.org> wrote:
> On Wed, Jun 6, 2018 at 9:23 AM Chang S. Bae <chang.seok.bae@intel.com> wrote:
>>
>> The new entry will be equivalent to that of x86-64 which
>> stores CPU number. The entry is placed in segment 23 in GDT
>> by bumping down 23-28 by one, which are all kernel-internal
>> segments and so have no impact on user space.
>>
>> CPU_NUMBER segment will always be at '%ss (USER_DS) + 80'
>> for the default (flat, initial) user space %ss.
>
> No, it won't :(  This is because, on Xen PV, user code very frequently
> sees a different, Xen-supplied "flat" SS value.  This is definitely
> true right now on 64-bit, and I'm reasonably confident it's also the
> case on 32-bit.
>
> As it stands, as far as I can tell, we don't have a "cpu number"
> segment on 32-bit kernels.  I see no compelling reason to add one, and
> we should definitely not add one as part of the FSGSBASE series.  I
> think the right solution is to rename the 64-bit segment to
> "CPU_NUMBER" and then have the rearrangement of the initialization
> code as a followup patch.  The goal is to make the patches
> individually reviewable.  As it stands, this patch adds some #defines
> without making them work, which is extra confusing.
>
> Given how many times we screwed it up, I really want the patch that
> moves the initialization of the 64-bit CPU number to be obviously
> correct and to avoid changing the sematics of anything except the
> actual CPU number fields during boot.
>
> So NAK to this patch, at least as part of the FSGSBASE series.
>
> (My apologies -- a bunch of this is because I along with everyone else
> misunderstood the existing code.)

The sole purpose of this segment is for the getcpu() function in the
VDSO.  No other userspace code can rely on its presence or location.

--
Brian Gerst