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
