Re: CPU Lockups in KVM with deferred hrtimer rearming

["Verma, Vishal L" <vishal.l.verma@intel.com>]

On Tue, 2026-04-21 at 09:39 +0200, Thomas Gleixner wrote:
> 
> Subject: entry: Enforce hrtimer rearming in the irqentry_exit path
> From: Thomas Gleixner <tglx@kernel.org>
> Date: Tue, 21 Apr 2026 09:00:52 +0200
> 
> irqentry_exit_to_kernel_mode_after_preempt() invokes
> hrtimer_rearm_deferred() only when the interrupted context had interrupts
> enabled. That's a correct decision because the timer interrupt can only be
> delivered in interrupt enabled contexts. The interrupt disabled path is
> used by exceptions and traps which never touch the hrtimer mechanics.
> 
> So much for the theory, but then there is VIRT which ruins everything.
> 
> KVM invokes regular interrupts with pt_regs which have interrupts
> disabled. That's correct from the KVM point of view, but completely
> violates the obviously correct expectations of the interrupt entry/exit
> code.
> 
> Cure this by adding a hrtimer_rearm_deferred() invocation into the
> interrupted context has interrupt disabled path of
> irqentry_exit_to_kernel_mode_after_preempt().
> 
> That's unfortunate when there is an actual reschedule pending, but it can't
> be avoided because KVM invokes a lot of code and also reenables interrupts
> _before_ reaching the point where the reschedule condition is handled. That
> can delay the rearming significantly, which in turn can cause artificial
> latencies.
> 
> Fixes: 0e98eb14814e ("entry: Prepare for deferred hrtimer rearming")
> Reported-by: "Verma, Vishal L" <vishal.l.verma@intel.com>
> Signed-off-by: Thomas Gleixner <tglx@kernel.org>
> Closes: https://lore.kernel.org/70cd3e97fbb796e2eb2ff8cd4b7614ada05a5f24.camel@intel.com

Hi Thomas, I tested this and verified it solves both the tests, no more
lockups. If this is the final fix, you can add:

Tested-by: Vishal Verma <vishal.l.verma@intel.com>

(I'm queueing up Peter's patch on the CI now too)

> ---
>  include/linux/irq-entry-common.h |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> --- a/include/linux/irq-entry-common.h
> +++ b/include/linux/irq-entry-common.h
> @@ -516,6 +516,14 @@ irqentry_exit_to_kernel_mode_after_preem
>  		instrumentation_end();
>  	} else {
>  		/*
> +		 * This is sadly required due to KVM, which invokes regular
> +		 * interrupt handlers with interrupt disabled state in @regs.
> +		 */
> +		instrumentation_begin();
> +		hrtimer_rearm_deferred();
> +		instrumentation_end();
> +
> +		/*
>  		 * IRQ flags state is correct already. Just tell RCU if it
>  		 * was not watching on entry.
>  		 */