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

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 24/10/2025 17:17, David Woodhouse wrote:
> 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@xxxxxxx>
>>>>> ---
>>>> 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

Thanks this is helpful! Unfortunately lazy_mmu is only used in the PV
case, so I'd need to run a PV guest. And the distro I'm using (Arch
Linux) does not have a Xen package :/ It can be built from source from
the AUR but that looks rather involved. Are there some prebuilt binaries
I could grab and just point QEMU to?

- Kevin