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

[David Hildenbrand (Red Hat)]

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.

-- 
Cheers

David