Re: [PATCH v3 1/3] x86/hyperv: Set irq_retrigger for root partition PCI/MSI irqchip

[Naman Jain <namjain@linux.microsoft.com>]

On 9/8/2026 1:23 AM, Michael Kelley wrote:
> From: Naman Jain <namjain@linux.microsoft.com> Sent: Sunday, September 6, 2026 11:51 PM
>>
>> The Hyper-V root partition vPCI MSI irqchip lost its irq_retrigger()
>> callback when it was converted to msi_create_parent_irq_domain(). The
>> callback was present on the original irqchip and was dropped during that
>> conversion.
>>
>> On CPU hot-unplug fixup_irqs() migrates the interrupts which are affine to
>> the outgoing CPU to a new target. If an interrupt still has its pending bit
>> set in the outgoing CPU's IRR at that point, fixup_irqs() resends it on the
>> new target through the irqchip's irq_retrigger() callback. As the root
>> partition PCI/MSI chip no longer provides that callback, the pending
>> interrupt is silently dropped, which can result in lost interrupts, stalls
>> and "No irq handler for vector" messages during CPU hotplug.
>>
>> Restore irq_chip_retrigger_hierarchy() as the irq_retrigger() callback for
>> the root partition PCI/MSI irqchip, so that a pending interrupt is resent
>> on its new target CPU via the parent x86 vector domain.
>>
>> Fixes: 4691db0704ac ("x86/hyperv: Switch to msi_create_parent_irq_domain()")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
>> ---
>>   arch/x86/hyperv/irqdomain.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/x86/hyperv/irqdomain.c b/arch/x86/hyperv/irqdomain.c
>> index b3ad50a874dca..1e273d7d6c5b3 100644
>> --- a/arch/x86/hyperv/irqdomain.c
>> +++ b/arch/x86/hyperv/irqdomain.c
>> @@ -304,6 +304,7 @@ static struct irq_chip hv_pci_msi_controller = {
>>   	.irq_ack		= irq_chip_ack_parent,
>>   	.irq_compose_msi_msg	= hv_irq_compose_msi_msg,
>>   	.irq_set_affinity	= irq_chip_set_affinity_parent,
>> +	.irq_retrigger		= irq_chip_retrigger_hierarchy,
>>   };
> 
> There's a slight discrepancy compared with the guest pci-hyperv.c driver,
> where setting the .irq_retrigger function is done dynamically in
> hv_pcie_init_dev_msi_info() instead of statically.  The parallel here would
> be to set it in hv_init_dev_msi_info(). But given that Patch 3 of series
> removes setting .irq_retrigger in both cases, the discrepancy shouldn't
> really matter.
> 
> If there's a reason to prefer one approach vs. the other, I don't know
> what that reason is.
> 
> Reviewed-by: Michael Kelley <mhklinux@outlook.com>

This change should be done for the outermost chip, i.e. in
hv_init_dev_msi_info(). With change 2, it won't matter, but patch 2 is
not being back ported. So, I'll fix it.

Regards,
Naman

> 
>>
>>   static bool hv_init_dev_msi_info(struct device *dev, struct irq_domain *domain,
>> --
>> 2.43.0
>>
>