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

["David Hildenbrand (Red Hat)" <david@xxxxxxxxxx>]

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