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Date:   Wed, 06 Jun 2018 15:53:02 -0700
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Subject: Re: [PATCH v2 7/8] x86/segments/32: Introduce CPU_NUMBER segment
To:     Brian Gerst <brgerst@gmail.com>, Andy Lutomirski <luto@kernel.org>
CC:     "Bae, Chang Seok" <chang.seok.bae@intel.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 June 6, 2018 12:07:15 PM PDT, Brian Gerst <brgerst@gmail.com> wrote:
>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

Unfortunately that is not true in reality :(
-- 
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