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

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Message-ID: <c377fab9-54f1-4eb9-8810-013a8bfb340e@siemens.com>
Date: Wed, 4 Feb 2026 08:19:59 +0100
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Wei Liu <wei.liu@...nel.org>
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 08:00, 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?

> Note we don't support RT yet, but if issues are found we might as well
> just fix them up.

This is actually not about RT itself but about supporting all
configurable locking semantics of the. And the mshv drivers fail here.

> 
> How urgent do you want this patch to get applied?

If I asked my folks: yesterday (we shipped it...).

We would also need in upstream stable and stable-rt, though it may not
reach the current production kernel anymore (6.1-rt from bookworm)
because it reaching EOL in a few months.

Jan

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