Re: [PATCH v2] x86/hotplug: Silence APIC only after all irq's are migrated

[Raj, Ashok]

Hi Thomas,

I was wondering if you got a chance to take a look at this fix?

I had some mail issues recently and they showed up at lore after 2
days. I wasn't sure if you got the original mail, or maybe it didn't
make it. 

If you had a different way to fix it, we can try those out. 


On Thu, Aug 20, 2020 at 05:42:03PM -0700, Ashok Raj wrote:
> When offlining CPUs, fixup_irqs() migrates all interrupts away from the
> outgoing CPU to an online CPU. It's always possible the device sent an
> interrupt to the previous CPU destination. Pending interrupt bit in IRR in
> LAPIC identifies such interrupts. apic_soft_disable() will not capture any
> new interrupts in IRR. This causes interrupts from device to be lost during
> CPU offline. The issue was found when explicitly setting MSI affinity to a
> CPU and immediately offlining it. It was simple to recreate with a USB
> ethernet device and doing I/O to it while the CPU is offlined. Lost
> interrupts happen even when Interrupt Remapping is enabled.
> 
> Current code does apic_soft_disable() before migrating interrupts.
> 
> native_cpu_disable()
> {
> 	...
> 	apic_soft_disable();
> 	cpu_disable_common();
> 	  --> fixup_irqs(); // Too late to capture anything in IRR.
> }
> 
> Just flipping the above call sequence seems to hit the IRR checks
> and the lost interrupt is fixed for both legacy MSI and when
> interrupt remapping is enabled.

On another note, we have tested both with and without the read
after write when programming MSI addr/data on the device. It didn't
seem to change the results. But I think its a useful one to add
for correctness.

https://lore.kernel.org/lkml/878si6rx7f.fsf@xxxxxxxxxxxxxxxxxxxxxxx/

This bug been eluding for a while. Looking for your feedback.

> 
> Fixes: 60dcaad5736f ("x86/hotplug: Silence APIC and NMI when CPU is dead")
> Link: https://lore.kernel.org/lkml/875zdarr4h.fsf@xxxxxxxxxxxxxxxxxxxxxxx/
> Reported-by: Evan Green <evgreen@xxxxxxxxxxxx>
> Tested-by: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
> Tested-by: Evan Green <evgreen@xxxxxxxxxxxx>
> Reviewed-by: Evan Green <evgreen@xxxxxxxxxxxx>
> Signed-off-by: Ashok Raj <ashok.raj@xxxxxxxxx>
> ---
> v2:
> - Typos and fixes suggested by Randy Dunlap
> 
> To: linux-kernel@xxxxxxxxxxxxxxx
> To: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Sukumar Ghorai <sukumar.ghorai@xxxxxxxxx>
> Cc: Srikanth Nandamuri <srikanth.nandamuri@xxxxxxxxx>
> Cc: Evan Green <evgreen@xxxxxxxxxxxx>
> Cc: Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>  arch/x86/kernel/smpboot.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 27aa04a95702..3016c3b627ce 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1594,13 +1594,20 @@ int native_cpu_disable(void)
>  	if (ret)
>  		return ret;
>  
> +	cpu_disable_common();
>  	/*
>  	 * Disable the local APIC. Otherwise IPI broadcasts will reach
>  	 * it. It still responds normally to INIT, NMI, SMI, and SIPI
> -	 * messages.
> +	 * messages. It's important to do apic_soft_disable() after
> +	 * fixup_irqs(), because fixup_irqs() called from cpu_disable_common()
> +	 * depends on IRR being set. After apic_soft_disable() CPU preserves
> +	 * currently set IRR/ISR but new interrupts will not set IRR.
> +	 * This causes interrupts sent to outgoing CPU before completion
> +	 * of IRQ migration to be lost. Check SDM Vol 3 "10.4.7.2 Local
> +	 * APIC State after It Has been Software Disabled" section for more
> +	 * details.
>  	 */
>  	apic_soft_disable();
> -	cpu_disable_common();
>  
>  	return 0;
>  }
> -- 
> 2.7.4
>