Re: [PATCH] x86: mshyperv: Use kthread for vmbus interrupts on PREEMPT_RT

[Wei Liu <wei.liu@kernel.org>]

On Wed, Feb 04, 2026 at 08:32:04AM +0100, Jan Kiszka wrote:
> On 04.02.26 08:29, Wei Liu wrote:
> > On Wed, Feb 04, 2026 at 08:26:48AM +0100, Jan Kiszka wrote:
> >> On 04.02.26 08:19, Jan Kiszka wrote:
> >>> On 04.02.26 08:00, Wei Liu wrote:
> >>>> On Tue, Feb 03, 2026 at 05:01:30PM +0100, Jan Kiszka wrote:
> >>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
> >>>>>
> >>>>> Resolves the following lockdep report when booting PREEMPT_RT on Hyper-V
> >>>>> with related guest support enabled:
> >>>>
> >>>> So all it takes to reproduce this is to enabled PREEMPT_RT?
> >>>>
> >>>
> >>> ...and enable CONFIG_PROVE_LOCKING so that you do not have to wait for
> >>> your system to actually run into the bug. Lockdep already triggers
> >>> during bootup.
> >>>
> >>>> Asking because ...
> >>>>
> >>>>>  	struct pt_regs *old_regs = set_irq_regs(regs);
> >>>>> @@ -158,8 +196,12 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_hyperv_callback)
> >>>>>  	if (mshv_handler)
> >>>>>  		mshv_handler();
> >>>>
> >>>> ... to err on the safe side we should probably do the same for
> >>>> mshv_handler as well.
> >>>>
> >>>
> >>> Valid question. We so far worked based on lockdep reports, and the
> >>> mshv_handler didn't trigger yet. Either it is not run in our setup, or
> >>> it is actually already fine. But I have a code review on my agenda
> >>> regarding potential remaining issues in mshv.
> >>>
> >>> Is there something needed to trigger the mshv_handler so that we can
> >>> test it?
> >>>
> >>
> >> Ah, that depends on CONFIG_MSHV_ROOT. Is that related to the accelerator
> >> mode that Magnus presented in [1]? We briefly chatted about it and also
> >> my problems with the drivers after his talk on Saturday.
> > 
> > Yes. That is the driver. If PROVE_LOCKING triggers the warning without
> > running the code, perhaps turning on MSHV_ROOT is enough.
> > 
> 
> But if my VM is not a root partition, I wouldn't use that driver, would I?

No, you wouldn't.  You cannot do that until later this year. If you
cannot test that, so be it. I'm fine with applying your patch and then
move the mshv_handler logic later ourselves.

I've CC'ed a few folks from Microsoft.

Saurabh, Long, and Dexuan, can you review and test this patch for VMBus?

Thanks,
Wei

> 
> Jan
> 
> -- 
> Siemens AG, Foundational Technologies
> Linux Expert Center
>