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

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 27/10/2025 13:29, David Hildenbrand wrote:
> 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@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?
>>>
>>> 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.

FWIW in that particular case it's relatively easy to tell this is
specific to Xen PV (this is only used in mmu_pv.c and enlighten_pv.c).
Knowing what to test is certainly not obvious in general, though.

- Kevin

>
> 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.