Re: [PATCH v3 11/13] x86/xen: use lazy_mmu_state when context-switching

David Woodhouse
2025-10-24T15:17:25Z

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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@arm.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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