[PATCH v9] um: Enable preemption in UML

[Benjamin Berg]

Hi,

On Wed, 2024-04-03 at 07:27 +0100, anton.ivanov at cambridgegreys.com
wrote:
> From: Anton Ivanov <anton.ivanov at cambridgegreys.com>
> 
> 1. Preemption requires saving/restoring FPU state. This patch
> adds support for it using GCC intrinsics as well as appropriate
> storage space in the thread structure. We reuse the space
> which is already allocated for the userspace threads in the
> thread_info structure.

Do we need to store the FPU state even? After all, we cannot simply
overwrite userspace FPU registers in UML.

Seriously wondering about that. It seems to me it would be sufficient
to ensure that only one kernel task is in a
kern_fpu_begin()/kern_fpu_end() section at any point in time.

Now, I don't know how preempt_disable()/preempt_enable() behaves
exactly. But I would assume it makes such a guarantee and then we can
simply map kern_fpu_begin to preempt_disable and kern_fpu_end to
preempt_enable.

> 2. irq critical sections need preempt_disable()/preempt_enable().
> 
> 3. TLB critical sections need preempt_disable()/preempt_enable().
> 
> 4. UML TLB flush is also invoked during a fork. This happens
> with interrupts and preempt disabled which disagrees with the
> standard mm locking via rwsem. The mm lock for this code path
> had to be replaced with an rcu.

Hopefully that flush during fork will be gone soon :-)

Benjamin

> [...]