On 12/11/2017 10:40 AM, Andy Lutomirski wrote: >> Also, from a high level, this does increase the overhead of KPTI in a >> non-trivial way, right? It costs us three more page table pages per >> process allocated at fork() and freed at exit() and a new TLB flush. > Yeah, but no one will care. modify_ldt() is used for DOSEMU, Wine, > and really old 32-bit programs.
The heavyweight part of map_ldt_struct() (and unmap) looks to run whenever we have KPTI enabled. I'm missing how it gets avoided for the non-DOSEMU cases. I thought there would be a "fast path" where we just use the normal clear_LDT() LDT from the cpu_entry_area and don't have to do any of this, but I'm missing where that happens. Do we need a check in (un)map_ldt_struct() for !mm->context.ldt? Just to make sure I understand this: We now have two places that LDTs live in virtual space: 1. The "plain" one that we get from clear_LDT() which lives in the cpu_entry_area. (No additional overhead when doing this) 2. The new one under the special PGD that's only used for modify_ldt() and is fairly slow. (plenty of overhead, but nobody cares).
