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

[Dave Hansen]

On 12/11/2017 11:39 AM, Andy Lutomirski wrote:
>> 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();
> }

I was looking at the map/unmap paths.  It looks to me like the cases
where there is map/unmap overhead, we *are* doing checking against
mm->context.ldt.  It just wasn't visible from the patch context.

In any case, it would be really nice to call that out if you revise
these in the patch description: none of these LDT acrobatics are used in
the common case.  Virtually every process uses the !ldt paths which
don't do any of this.