Re: [PATCH PTI v2 6/6] x86/pti: Put the LDT in its own PGD if PTI is on

[Dave Hansen <dave.hansen@intel.com>]

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).