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

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Message-ID: <09c72960-dbc1-4ca8-a0f8-aed686de4f19@siemens.com>
Date: Wed, 4 Feb 2026 14:04:57 +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>, skinsburskii@...ux.microsoft.com,
 mrathor@...ux.microsoft.com, anirudh@...rudhrb.com,
 schakrabarti@...ux.microsoft.com, ssengar@...ux.microsoft.com
Subject: Re: [PATCH] x86: mshyperv: Use kthread for vmbus interrupts on
 PREEMPT_RT

On 04.02.26 08:36, Wei Liu wrote:
> 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@...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?
> 
> 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.

Based on my review, I bet you have to lift the mshv_handler to a thread
as well, e.g. sysvec_hyperv_callback ... -> kick_vp -> wake_up.

OTOH, the probability that someone tries to use PREEMPT_RT as root Linux
is likely even lower than someone trying it as a normal guest. IMHO, you
could also consider to rule out that combination at Kconfig level.

Jan

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