On 2018-09-19 19:00:34 [+0200], Paolo Bonzini wrote: > On 19/09/2018 18:57, Sebastian Andrzej Siewior wrote: > > On 2018-09-19 07:55:51 [+0200], Paolo Bonzini wrote: > >> A kthread can do use_mm/unuse_mm. > > > > indeed. The FPU struct for the kernel thread isn't valid / does not > > contain the expected PKRU value. So loading the pkru value from the > > struct FPU does not work as expected. We could set it to 0 for a kernel > > thread so we don't end up with a random value. > > If we want to get this usecase working then we would have to move pkru > > value from FPU to mm_struct and consider it in use_mm(). Do we want > > this? > > As a start, I think keeping it in the FPU struct but loading it > unconditionally will work. kthreads will not obey PKU but it will be > better already. Are you saying I should load the uninitialized value for kthreads or load 0 to have in a known state? > I honestly don't know if PKRU should be per-mm, I don't know mm very > well despite my brilliant observation above. :) Now that I have qemu machine with PKRU I would need to figure out how this works. So unless I am mistaken mm is per task and the FPU is per thread. And since the FPU struct isn't initialized for kthreads, we should end up with 0. But setting to 0 if not used sounds good. > Paolo Sebastian
