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

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Message-ID: <7324616a8d2631aa8132f725f9f6551e3e6b21dd.camel@infradead.org>
Date: Fri, 24 Oct 2025 16:17:20 +0100
From: David Woodhouse <dwmw2@...radead.org>
To: Kevin Brodsky <kevin.brodsky@....com>, 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>, "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 Fri, 2025-10-24 at 17:05 +0200, Kevin Brodsky wrote:
> On 24/10/2025 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?
> 
> I considered boot-testing a Xen guest (considering the Xen-specific
> changes in this series), but having no idea how to go about it I quickly
> gave up... Happy to follow instructions :)

https://qemu-project.gitlab.io/qemu/system/i386/xen.html covers booting
Xen HVM guests, and near the bottom PV guests too (for which you do
need a copy of Xen to run in QEMU with '--kernel xen', and your
distro's build should suffice for that).

Let me know if you have any trouble. Here's a sample command line which
works here...

qemu-system-x86_64 -display none --accel kvm,xen-version=0x40011,kernel-irqchip=split -drive file=/var/lib/libvirt/images/fedora28.qcow2,if=xen -kernel ~/git/linux-2.6/arch/x86/boot/bzImage -append "root=/dev/xvda1 console=ttyS0" -serial mon:stdio

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