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

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Message-ID: <20260204070004.GM79272@liuwe-devbox-debian-v2.local>
Date: Wed, 4 Feb 2026 07:00:04 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Jan Kiszka <jan.kiszka@...mens.com>
Cc: "K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, 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 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?

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.

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

How urgent do you want this patch to get applied?

Thanks,
Wei

>  
> -	if (vmbus_handler)
> -		vmbus_handler();
> +	if (vmbus_handler) {
> +		if (IS_ENABLED(CONFIG_PREEMPT_RT))
> +			vmbus_irqd_wake();
> +		else
> +			vmbus_handler();
> +	}
>  
>  	if (ms_hyperv.hints & HV_DEPRECATING_AEOI_RECOMMENDED)
>  		apic_eoi();
> @@ -174,6 +216,10 @@ void hv_setup_mshv_handler(void (*handler)(void))
>  
>  void hv_setup_vmbus_handler(void (*handler)(void))
>  {
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && !vmbus_irq_initialized) {
> +		BUG_ON(smpboot_register_percpu_thread(&vmbus_irq_threads));
> +		vmbus_irq_initialized = true;
> +	}
>  	vmbus_handler = handler;
>  }
>  
> @@ -181,6 +227,8 @@ void hv_remove_vmbus_handler(void)
>  {
>  	/* We have no way to deallocate the interrupt gate */
>  	vmbus_handler = NULL;
> +	smpboot_unregister_percpu_thread(&vmbus_irq_threads);
> +	vmbus_irq_initialized = false;
>  }
>  
>  /*
> -- 
> 2.51.0