[PATCH v4 11/12] x86/xen: use lazy_mmu_state when context-switching

[Kevin Brodsky]

On 03/11/2025 19:23, David Hildenbrand (Red Hat) wrote:
> On 03.11.25 19:29, Kevin Brodsky wrote:
>> On 03/11/2025 16:15, David Hildenbrand (Red Hat) wrote:
>>> On 29.10.25 11:09, Kevin Brodsky wrote:
>>>> [...]
>>>>
>>>> @@ -437,7 +436,7 @@ static void xen_end_context_switch(struct
>>>> task_struct *next)
>>>>          xen_mc_flush();
>>>>        leave_lazy(XEN_LAZY_CPU);
>>>> -    if (test_and_clear_ti_thread_flag(task_thread_info(next),
>>>> TIF_LAZY_MMU_UPDATES))
>>>> +    if (next->lazy_mmu_state.active)
>>>
>>> This is nasty. If in_lazy_mmu_mode() is not sufficient, we will want
>>> to have a separate helper that makes it clear what the difference
>>> between both variants is.
>>
>> in_lazy_mmu_mode() operates on current, but here we're operating on a
>> different task. The difference is more fundamental than just passing a
>> task_struct * or not: in_lazy_mmu_mode() is about whether we're
>> currently in lazy MMU mode, i.e. not paused and not in interrupt
>> context. A task that isn't scheduled is never in lazy MMU mode -
>> lazy_mmu_state.active is just the saved state to be restored when
>> scheduled again.
>>
>> My point here is that we could have a helper for this use-case, but it
>> should not be used in other situations (at least not on current). Maybe
>> __task_lazy_mmu_active(task)? I do wonder if accessing lazy_mmu_state
>> directly isn't expressing the intention well enough though (checking the
>> saved state).
>
>
> Likely there should be a
>
> /**
>  * task_lazy_mmu_active - test whether the lazy-mmu mode is active for a
>  *              task
>  * @task: ...
>  *
>  * The lazy-mmu mode is active if a task has lazy-mmu mode enabled and
>  * currently not paused.
>  */
> static inline bool task_lazy_mmu_active(struct task_struct *task)
> {
>     return task->lazy_mmu_state.active;
> }
>
> /**
>  * in_lazy_mmu_mode() - test whether current is in lazy-mmu mode
>  *
>  * Test whether the current task is in lazy-mmu mode: whether the
>  * interrupts are enabled and the lazy-mmu mode is active for the
>  * current task.
>  */
>  static inline bool in_lazy_mmu_mode(void)
>  {
> +    if (in_interrupt())
> +        return false;
> +
>      return task_lazy_mmu_active(current);
>  }
>
>
> Something like that. Maybe we can find better terminology.

That's probably the clearest yes, will make the change. I can't think of
more self-documenting names, spelling out the difference in the comments
is likely the best we can do.

- Kevin