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

[Andy Lutomirski]

On Mon, Dec 11, 2017 at 11:47 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> 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.

Will do.

I'm currently fighting with the 5 level case.  I need to reorganize
the memory map a bit, but it's blowing up, and I'm not sure why yet.