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

[Jürgen Groß <jgross@xxxxxxxx>]

Please disregard this patch. It isn't fixing the real problem.

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.
>
> 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) {}

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