Re: [RFC 00/10] Process-local memory allocations for hiding KVM secrets

[Dave Hansen]

On 6/13/19 9:13 AM, Andy Lutomirski wrote:
>> It might make sense to use it for kmap_atomic() for debug purposes, as
>> it ensures that other users can no longer access the same mapping
>> through the linear map. However, it does come at quite a big cost, as we
>> need to shoot down the TLB of all other threads in the system. So I'm
>> not sure it's of general value?
> What I meant was that kmap_atomic() could use mm-local memory so that
> it doesn't need to do a global shootdown.  But I guess it's not
> actually used for real on 64-bit, so this is mostly moot.  Are you
> planning to support mm-local on 32-bit?

Do we *do* global shootdowns on kmap_atomic()s on 32-bit?  I thought we
used entirely per-cpu addresses, so a stale entry from another CPU can
get loaded in the TLB speculatively but it won't ever actually get used.
 I think it goes:

kunmap_atomic() ->
__kunmap_atomic() ->
kpte_clear_flush() ->
__flush_tlb_one_kernel() ->
__flush_tlb_one_user() ->
__native_flush_tlb_one_user() ->
invlpg

The per-cpu address calculation is visible in kmap_atomic_prot():

        idx = type + KM_TYPE_NR*smp_processor_id();