linux-kernel - Re: [PATCH v4 02/12] x86/xen: simplify flush_lazy

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Message-ID: <5a3ccb7e-9d36-4ac8-9634-c8dec3d6a47c@redhat.com>
Date: Sat, 1 Nov 2025 13:14:21 +0100
From: David Hildenbrand <david@...hat.com>
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 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 02/12] x86/xen: simplify flush_lazy_mmu()

On 29.10.25 11:08, Kevin Brodsky wrote:
> arch_flush_lazy_mmu_mode() is called when outstanding batched
> pgtable operations must be completed immediately. There should
> however be no need to leave and re-enter lazy MMU completely. The
> only part of that sequence that we really need is xen_mc_flush();
> call it directly.
> 
> Signed-off-by: Kevin Brodsky <kevin.brodsky@....com>
> ---
>   arch/x86/xen/mmu_pv.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 2a4a8deaf612..7a35c3393df4 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -2139,10 +2139,8 @@ static void xen_flush_lazy_mmu(void)
>   {
>   	preempt_disable();
>   
> -	if (xen_get_lazy_mode() == XEN_LAZY_MMU) {
> -		arch_leave_lazy_mmu_mode();
> -		arch_enter_lazy_mmu_mode();
> -	}
> +	if (xen_get_lazy_mode() == XEN_LAZY_MMU)
> +		xen_mc_flush();
>   
>   	preempt_enable();
>   }

Looks like that was moved to XEN code in

commit a4a7644c15096f57f92252dd6e1046bf269c87d8
Author: Juergen Gross <jgross@...e.com>
Date:   Wed Sep 13 13:38:27 2023 +0200

     x86/xen: move paravirt lazy code

And essentially the previous implementation lived in 
arch/x86/kernel/paravirt.c:paravirt_flush_lazy_mmu(void) in an 
implementation-agnostic way:

void paravirt_flush_lazy_mmu(void)
{
        preempt_disable();

        if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_MMU) {
                arch_leave_lazy_mmu_mode();
                arch_enter_lazy_mmu_mode();
        }

        preempt_enable();
}

So indeed, I assume just doing the flush here is sufficient.

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb