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

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Message-ID: <3ff4aaeb-61ce-4b72-ba90-1b66374b1b95@gmail.com>
Date: Fri, 24 Oct 2025 18:52:50 -0400
From: Demi Marie Obenour <demiobenour@...il.com>
To: David Hildenbrand <david@...hat.com>,
 David Woodhouse <dwmw2@...radead.org>, 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>, "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 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?
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
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