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

From: "David Hildenbrand (Red Hat)" <[email-masked]>
Date: Mon, 3 Nov 2025 17:15:18 +0100

On 29.10.25 11:09, Kevin Brodsky wrote:
> We currently set a TIF flag when scheduling out a task that is in
> lazy MMU mode, in order to restore it when the task is scheduled
> again.
> 
> The generic lazy_mmu layer now tracks whether a task is in lazy MMU
> mode in task_struct::lazy_mmu_state. We can therefore check that
> state when switching to the new task, instead of using a separate
> TIF flag.
> 
> Signed-off-by: Kevin Brodsky <[email-masked]>
> ---
>   arch/x86/include/asm/thread_info.h | 4 +---
>   arch/x86/xen/enlighten_pv.c        | 3 +--
>   2 files changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index e71e0e8362ed..0067684afb5b 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -100,8 +100,7 @@ struct thread_info {
>   #define TIF_FORCED_TF		24	/* true if TF in eflags artificially */
>   #define TIF_SINGLESTEP		25	/* reenable singlestep on user return*/
>   #define TIF_BLOCKSTEP		26	/* set when we want DEBUGCTLMSR_BTF */
> -#define TIF_LAZY_MMU_UPDATES	27	/* task is updating the mmu lazily */
> -#define TIF_ADDR32		28	/* 32-bit address space on 64 bits */
> +#define TIF_ADDR32		27	/* 32-bit address space on 64 bits */
>   
>   #define _TIF_SSBD		BIT(TIF_SSBD)
>   #define _TIF_SPEC_IB		BIT(TIF_SPEC_IB)
> @@ -114,7 +113,6 @@ struct thread_info {
>   #define _TIF_FORCED_TF		BIT(TIF_FORCED_TF)
>   #define _TIF_BLOCKSTEP		BIT(TIF_BLOCKSTEP)
>   #define _TIF_SINGLESTEP		BIT(TIF_SINGLESTEP)
> -#define _TIF_LAZY_MMU_UPDATES	BIT(TIF_LAZY_MMU_UPDATES)
>   #define _TIF_ADDR32		BIT(TIF_ADDR32)
>   
>   /* flags to check in __switch_to() */
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 4806cc28d7ca..f40f5999352e 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -426,7 +426,6 @@ static void xen_start_context_switch(struct task_struct *prev)
>   
>   	if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU) {
>   		arch_leave_lazy_mmu_mode();
> -		set_ti_thread_flag(task_thread_info(prev), TIF_LAZY_MMU_UPDATES);
>   	}
>   	enter_lazy(XEN_LAZY_CPU);
>   }
> @@ -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.

-- 
Cheers

David