On Mon, Dec 11, 2017 at 11:32 AM, Dave Hansen <dave.han...@intel.com> wrote:
> 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.
It doesn't get called unless modify_ldt() is used.
>
> 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?
I'm confused.
if (unlikely(ldt)) {
do something slowish;
} else {
clear_LD();
}
>
> 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).
Yes.
