Messages in this thread | | | Subject | Re: [PATCH 00/11] Use global pages with PTI | From | Dave Hansen <> | Date | Fri, 23 Mar 2018 17:40:57 -0700 |
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On 03/23/2018 11:26 AM, Linus Torvalds wrote: > On Fri, Mar 23, 2018 at 10:44 AM, Dave Hansen > <dave.hansen@linux.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.
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