linux-kernel - Re: [PATCH 00/11] Use global pages with PTI

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Date:   Fri, 23 Mar 2018 17:40:57 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jürgen Groß <jgross@...e.com>,
        the arch/x86 maintainers <x86@...nel.org>, namit@...are.com
Subject: Re: [PATCH 00/11] Use global pages with PTI

On 03/23/2018 11:26 AM, Linus Torvalds wrote:
> On Fri, Mar 23, 2018 at 10:44 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
>>
>> This adds one major change from the last version of the patch set
>> (present in the last patch).  It makes all kernel text global for non-
>> PCID systems.  This keeps kernel data protected always, but means that
>> it will be easier to find kernel gadgets via meltdown on old systems
>> without PCIDs.  This heuristic is, I think, a reasonable one and it
>> keeps us from having to create any new pti=foo options
> 
> Sounds sane.
> 
> The patches look reasonable, but I hate seeing a patch series like
> this where the only ostensible reason is performance, and there are no
> performance numbers anywhere..

Well, rats.  This somehow makes things slower with PCIDs on.  I thought
I reversed the numbers, but I actually do a "grep -c GLB
/sys/kernel/debug/page_tables/kernel" and record that in my logs right
next to the output of time(1), so it's awfully hard to screw up.

This is time doing a modestly-sized kernel compile on a 4-core Skylake
desktop.

                        User Time       Kernel Time     Clock Elapsed
Baseline ( 0 GLB PTEs)  803.79          67.77           237.30
w/series (28 GLB PTEs)  807.70 (+0.7%)  68.07 (+0.7%)   238.07 (+0.3%)

Without PCIDs, it behaves the way I would expect.

I'll ask around, but I'm open to any ideas about what the heck might be
causing this.