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

[Dave Hansen]

On 03/23/2018 11:26 AM, Linus Torvalds wrote:
> On Fri, Mar 23, 2018 at 10:44 AM, Dave Hansen
> <dave.hansen@xxxxxxxxxxxxxxx> 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.