linux-kernel - Re: [PATCH] x86: mshyperv: Use kthread for vmbus interrupts on PREEMPT

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Message-ID: <d1dded05-a47f-4be2-94c4-913104c758e2@siemens.com>
Date: Wed, 4 Feb 2026 08:32:04 +0100
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Wei Liu <wei.liu@...nel.org>
Cc: Magnus Kulke <magnuskulke@...ux.microsoft.com>,
 "K. Y. Srinivasan" <kys@...rosoft.com>,
 Haiyang Zhang <haiyangz@...rosoft.com>, Dexuan Cui <decui@...rosoft.com>,
 Long Li <longli@...rosoft.com>, Thomas Gleixner <tglx@...nel.org>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
 Florian Bezdeka <florian.bezdeka@...mens.com>,
 RT <linux-rt-users@...r.kernel.org>, Mitchell Levy <levymitchell0@...il.com>
Subject: Re: [PATCH] x86: mshyperv: Use kthread for vmbus interrupts on
 PREEMPT_RT

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@...mens.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?

Jan

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