> On Aug 20, 2019, at 6:57 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > > On 8/20/19 12:51 AM, Song Liu wrote: >> In our x86_64 kernel, pti_clone_pgtable() fails to clone 7 PMDs because >> of this issuse, including PMD for the irq entry table. For a memcache >> like workload, this introduces about 4.5x more iTLB-load and about 2.5x >> more iTLB-load-misses on a Skylake CPU. > > I was surprised that this manifests as a performance issue. Usually > messing up PTI page table manipulation means you get to experience the > jobs of debugging triple faults. But, it makes sense if its this line: > > /* > * Note that this will undo _some_ of the work that > * pti_set_kernel_image_nonglobal() did to clear the > * global bit. > */ > pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE); > > which is restoring the Global bit. > > *But*, that shouldn't get hit on a Skylake CPU since those have PCIDs > and shouldn't have a global kernel image. Could you confirm whether > PCIDs are supported on this CPU? Yes, pcid is listed in /proc/cpuinfo. Thanks, Song
