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

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Message-ID: <bcc78ea0-5eca-49e5-bafd-84a16e06ab98@arm.com>
Date: Mon, 3 Nov 2025 18:06:24 +0000
From: Kevin Brodsky <kevin.brodsky@....com>
To: David Hildenbrand <david@...hat.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 01/11/2025 12:14, David Hildenbrand wrote:
> 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();
> }

Indeed, I saw that too. Calling the generic leave/enter functions made
some sense at that point, but now that the implementation is
Xen-specific we can directly call xen_mc_flush().

>
> So indeed, I assume just doing the flush here is sufficient.
>
> Reviewed-by: David Hildenbrand <david@...hat.com> 

Thanks for the review!

- Kevin