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

David Hildenbrand
2025-10-27T12:29:30Z

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@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?
>>
>> 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.

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.

--
Cheers

David / dhildenb