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

[Naman Jain]

On 9/8/2026 1:23 AM, Michael Kelley wrote:
> From: Naman Jain <namjain@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Naman Jain <namjain@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   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@xxxxxxxxxxx>

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