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

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Message-ID: <10ec70f2-27a5-477f-b6e9-164f7b7545d9@siemens.com>
Date: Wed, 4 Feb 2026 08:26:48 +0100
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Wei Liu <wei.liu@...nel.org>,
 Magnus Kulke <magnuskulke@...ux.microsoft.com>
Cc: "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 08:19, 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.

Jan

[1]
https://fosdem.org/2026/schedule/event/BFQ8XA-introducing-mshv-accelerator-in-qemu/

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