[RFC PATCH 2/9] genirq: Allow the state of a forwarded irq to be save/restored

Hi Marc,

On Wed, Jun 25, 2014 at 10:28:43AM +0100, Marc Zyngier wrote:

When a peripheral is shared between virtual machines, its interrupt
state becomes part of the guest's state, and must be switched accordingly.

Introduce a pair of accessors (irq_get_fwd_state/irq_set_fwd_state) to
retrieve the bits that can be of interest to KVM: pending, active, and masked.

- irq_get_fwd_state returns the state of the interrupt according to a mask
containing any of the IRQ_STATE_PENDING, IRQ_STATE_ACTIVE or IRQ_STATE_MASKED
bits.
- irq_set_fwd_state sets the state of the interrupt according to a similar mask.

Only a forwarded interrupt can be manipulated in such a way.

[...]

+/**
+ *	irq_set_fwd_state - set the state of a forwarded interrupt.
+ *	@irq: Interrupt line that is forwarded to a VM
+ *	@val: State to be restored
+ *	@mask: Bitmask of IRQ_FWD_STATE_* defining the valid bits in @val
+ *
+ *	This call sets the state of a forwarded interrupt, depending
+ *	on the mask which indicates the valid bits.
+ *
+ *	This function should be called with preemption disabled if the
+ *	interrupt controller has per-cpu registers.
+ */
+int irq_set_fwd_state(unsigned int irq, u32 val, u32 mask)
+{
+	struct irq_desc *desc;
+	struct irq_data *data;
+	struct irq_chip *chip;
+	unsigned long flags;
+
+	desc = irq_to_desc(irq);
+	if (!desc)
+		return -EINVAL;
+
+	data = irq_desc_get_irq_data(desc);
+	if (!irqd_irq_forwarded(data))
+		return -EINVAL;
+
+	chip = irq_desc_get_chip(desc);
+	if (!chip->irq_set_fwd_state)
+		return -EINVAL;
+
+	chip_bus_lock(desc);
+	raw_spin_lock_irqsave(&desc->lock, flags);
+	chip->irq_set_fwd_state(data, val, mask);
+	raw_spin_unlock_irqrestore(&desc->lock, flags);
+	chip_bus_sync_unlock(desc);

Having looked at this, I don't think you need to take the desc->lock after
all. I thought you might need it for irq_desc_get_chip, but that's not going
to change dynamically and you'd end up with a lock ordering problem against
chip_bus_lock anyway.

Will