linux-kernel - Re: [PATCH v3 11/13] x86/xen: use lazy_mmu

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Message-ID: <2be0732d-d8fb-424d-bc4e-443cc49d0092@arm.com>
Date: Mon, 27 Oct 2025 14:32:42 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: David Hildenbrand <david@...hat.com>,
 Demi Marie Obenour <demiobenour@...il.com>,
 David Woodhouse <dwmw2@...radead.org>, 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>, "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 v3 11/13] x86/xen: use lazy_mmu_state when
 context-switching

On 27/10/2025 13:29, David Hildenbrand wrote:
> On 25.10.25 00:52, Demi Marie Obenour wrote:
>> On 10/24/25 10:51, David Hildenbrand wrote:
>>> On 24.10.25 16:47, David Woodhouse wrote:
>>>> On Thu, 2025-10-23 at 22:06 +0200, David Hildenbrand wrote:
>>>>> On 15.10.25 10:27, 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>
>>>>>> ---
>>>>>
>>>>>
>>>>> Looks ok to me, but I hope we get some confirmation from x86 / xen
>>>>> folks.
>>>>
>>>>
>>>> I know tglx has shouted at me in the past for precisely this reminder,
>>>> but you know you can test Xen guests under QEMU/KVM now and don't need
>>>> to actually run Xen? Has this been boot tested?
>>>
>>> And after that, boot-testing sparc as well? :D
>>>
>>> If it's easy, why not. But other people should not suffer for all the
>>> XEN hacks we keep dragging along.
>>
>> Which hacks?  Serious question.  Is this just for Xen PV or is HVM
>> also affected?
>
> In the context of this series, XEN_LAZY_MMU.

FWIW in that particular case it's relatively easy to tell this is
specific to Xen PV (this is only used in mmu_pv.c and enlighten_pv.c).
Knowing what to test is certainly not obvious in general, though.

- Kevin

>
> Your question regarding PV/HVM emphasizes my point: how is a submitter
> supposed to know which XEN combinations to test (and how to test
> them), to not confidentially break something here.
>
> We really need guidance+help from the XEN folks here.