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

[Thomas Gleixner]

On Fri, 23 Mar 2018, Dave Hansen wrote:
> 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.

What's the performance benefit on !PCID systems? And I mean systems which
actually do not have PCID, not a PCID system with 'nopcid' on the command
line.

Thanks,

	tglx