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

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