Re: [PATCH] x86/xen: Fix lazy mmu handling across context switch

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 08/05/2026 10:33, Jürgen Groß wrote:
> Please disregard this patch. It isn't fixing the real problem.

That's what I would expect, see below.

>
> On 08.05.26 10:05, Juergen Gross wrote:
>> The recent rework of mmu lazy mode has resulted in problems when
>> running as a Xen PV guest. Enabling lazy mmu mode for the new context
>> during context switch is done from the arch_end_context_switch() hook,
>> but when calling this hook current hasn't been changed yet, so the
>> lazy mmu mode state of the wrong task is modified.

Currently xen_end_context_switch() checks if next has lazy MMU mode
enabled and if so calls arch_enter_lazy_mmu_mode(), i.e.
enter_lazy(XEN_LAZY_MMU). This does *not* modify any task state, rather
it writes to the xen_lazy_mode percpu variable.

I've thought about this from various angles when reworking lazy MMU, and
the conclusion I made is that arch_{start,end}_context_switch() have no
reason to change any task state. On arm64, for instance, we do nothing
at all on context switching, since everything lazy MMU-related is
tracked in task_struct and therefore already switched.

Xen is trickier because it tracks lazy MMU/CPU state in a percpu
variable, so these hooks do need to do something about it. This is
entirely Xen-internal though, and there's no reason to be calling
generic functions like lazy_mmu_mode_pause() that modify task state.

The idea behind commit 291b3abed657 ("x86/xen: use lazy_mmu_state when
context-switching") is that TIF_LAZY_MMU_UPDATES now duplicates
lazy_mmu_state in task_struct and we can therefore replace the former
with the latter. More specifically, the assumption is that
TIF_LAZY_MMU_UPDATES is set if and only if the task has been scheduled
out and __task_lazy_mmu_mode_active(task) is true.

Clearly there is something wrong with this assumption, but I still can't
put my finger on it. For now I would suggest reverting this commit if
that solves the issue Marek reported; the intention was not to introduce
any functional change, but only a (minor) optimisation.

- Kevin

>>
>> Additionally it is much cleaner to use lazy_mmu_mode_pause() and
>> lazy_mmu_mode_resume() in the Xen context switch hooks, as it avoids
>> conditionals in those hooks.
>>
>> In order not having to add another hook to be called after switching
>> current, modify lazy_mmu_mode_resume() to use a new sub-function which
>> takes a task pointer as parameter. This new sub-function can then be
>> used in the xen_end_context_switch() hook.
>>
>> Fixes: 291b3abed657 ("x86/xen: use lazy_mmu_state when
>> context-switching")
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>> ---
>>   arch/x86/xen/enlighten_pv.c |  7 ++-----
>>   include/linux/pgtable.h     | 33 ++++++++++++++++++++++++---------
>>   2 files changed, 26 insertions(+), 14 deletions(-)
>>
>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
>> index ed2d7a3756ce..67bb6bf6d240 100644
>> --- a/arch/x86/xen/enlighten_pv.c
>> +++ b/arch/x86/xen/enlighten_pv.c
>> @@ -424,9 +424,7 @@ static void xen_start_context_switch(struct
>> task_struct *prev)
>>   {
>>       BUG_ON(preemptible());
>>   -    if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU) {
>> -        arch_leave_lazy_mmu_mode();
>> -    }
>> +    lazy_mmu_mode_pause();
>>       enter_lazy(XEN_LAZY_CPU);
>>   }
>>   @@ -436,8 +434,7 @@ static void xen_end_context_switch(struct
>> task_struct *next)
>>         xen_mc_flush();
>>       leave_lazy(XEN_LAZY_CPU);
>> -    if (__task_lazy_mmu_mode_active(next))
>> -        arch_enter_lazy_mmu_mode();
>> +    lazy_mmu_mode_resume_task(next);
>>   }
>>     static unsigned long xen_store_tr(void)
>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
>> index cdd68ed3ae1a..83a099bf2038 100644
>> --- a/include/linux/pgtable.h
>> +++ b/include/linux/pgtable.h
>> @@ -326,6 +326,28 @@ static inline void lazy_mmu_mode_pause(void)
>>           arch_leave_lazy_mmu_mode();
>>   }
>>   +/**
>> + * lazy_mmu_mode_resume_task() - Resume the lazy MMU mode for a
>> specific task.
>> + *
>> + * Like lazy_mmu_mode_resume() below, but with a task specified.
>> + * Must be called only by lazy_mmu_mode_resume() or during context
>> switch.
>> + * Must never be called in interrupt context.
>> + *
>> + * Must match a call to lazy_mmu_mode_pause().
>> + *
>> + * Has no effect if called:
>> + * - While paused (inside another pause()/resume() pair)
>> + */
>> +static inline void lazy_mmu_mode_resume_task(struct task_struct *task)
>> +{
>> +    struct lazy_mmu_state *state = &task->lazy_mmu_state;
>> +
>> +    VM_WARN_ON_ONCE(state->pause_count == 0);
>> +
>> +    if (--state->pause_count == 0 && state->enable_count > 0)
>> +        arch_enter_lazy_mmu_mode();
>> +}
>> +
>>   /**
>>    * lazy_mmu_mode_resume() - Resume the lazy MMU mode.
>>    *
>> @@ -341,15 +363,8 @@ static inline void lazy_mmu_mode_pause(void)
>>    */
>>   static inline void lazy_mmu_mode_resume(void)
>>   {
>> -    struct lazy_mmu_state *state = &current->lazy_mmu_state;
>> -
>> -    if (in_interrupt())
>> -        return;
>> -
>> -    VM_WARN_ON_ONCE(state->pause_count == 0);
>> -
>> -    if (--state->pause_count == 0 && state->enable_count > 0)
>> -        arch_enter_lazy_mmu_mode();
>> +    if (!in_interrupt())
>> +        lazy_mmu_mode_resume_task(current);
>>   }
>>   #else
>>   static inline void lazy_mmu_mode_enable(void) {}
>