Re: [PATCH v4 1/4] x86/hyperv: Set irq_retrigger for root partition PCI/MSI irqchip

Naman Jain [off-list ref] writes:

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
Reviewed-by: Michael Kelley <redacted>
Signed-off-by: Naman Jain <redacted>

Reviewed-by: Nam Cao <redacted>