linux-kernel - Re: [PATCH v4 11/12] x86/xen: use lazy_mmu

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Message-ID: <c7c8a233-2103-4b48-b65e-ec81666d20e4@kernel.org>
Date: Mon, 3 Nov 2025 17:15:18 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Kevin Brodsky <kevin.brodsky@....com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Alexander Gordeev <agordeev@...ux.ibm.com>,
 Andreas Larsson <andreas@...sler.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>, Borislav Petkov
 <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 David Hildenbrand <david@...hat.com>, "David S. Miller"
 <davem@...emloft.net>, David Woodhouse <dwmw2@...radead.org>,
 "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
 Jann Horn <jannh@...gle.com>, Juergen Gross <jgross@...e.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Michal Hocko <mhocko@...e.com>,
 Mike Rapoport <rppt@...nel.org>, Nicholas Piggin <npiggin@...il.com>,
 Peter Zijlstra <peterz@...radead.org>, Ryan Roberts <ryan.roberts@....com>,
 Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Vlastimil Babka <vbabka@...e.cz>,
 Will Deacon <will@...nel.org>, Yeoreum Yun <yeoreum.yun@....com>,
 linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
 sparclinux@...r.kernel.org, xen-devel@...ts.xenproject.org, x86@...nel.org
Subject: Re: [PATCH v4 11/12] x86/xen: use lazy_mmu_state when
 context-switching

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 <kevin.brodsky@....com>
> ---
>   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