On 2019-01-18 13:17:28 [-0800], Dave Hansen wrote: > On 1/18/19 1:14 PM, Sebastian Andrzej Siewior wrote: > > The kernel saves task's FPU registers on user's signal stack before > > entering the signal handler. Can we avoid that and have in-kernel memory > > for that? Does someone rely on the FPU registers from the task in the > > signal handler? > > This is part of our ABI for *sure*. I missed that part. I will try to look it up and look see if says something about optional part. But ABI means we must keep doing it even if there are no users? > Inspecting that state is how > userspace makes sense of MPX or protection keys faults. We even use > this in selftests/. Okay. MPX does not check for FP_XSTATE_MAGIC[12] and simply assumes it is there. That is why I didn't find it. So we would break MPX. But then MPX is on its way out, so… Sebastian
