genirq: Update effective affinity for redirected interrupts

For redirected interrupts, irq_chip_redirect_set_affinity() does not
update the effective affinity mask, which then triggers the warning in
irq_validate_effective_affinity(). Also, because the effective affinity
mask is empty, the cpumask_test_cpu(smp_processor_id(), m) condition in
demux_redirect_remote() is always false, and the interrupt is always
redirected, even if it's already running on the target CPU.

Set the effective affinity mask to be the same as the requested affinity
mask. It's worth noting that irq_do_set_affinity() filters out offline
CPUs before calling chip->irq_set_affinity() (unless `force` is set), so
the mask passed to irq_chip_redirect_set_affinity() is already filtered.

The solution is not ideal because it may lie about the effective
affinity of the demultiplexed ("child") interrupt. If the requested
affinity mask includes multiple CPUs, the effective affinity, in
reality, is the intersection between the requested mask and the
demultiplexing ("parent") interrupt's effective affinity mask, plus
the first CPU in the requested mask.

Accurately describing the effective affinity of the demultiplexed
interrupt is not trivial because it requires keeping track of the
demultiplexing interrupt's effective affinity. That is tricky in the
context of CPU hot(un)plugging, where interrupt migration ordering is
not guaranteed. The solution in the initial version of the fixed patch,
which stored the first CPU of the demultiplexing interrupt's effective
affinity in the `target_cpu` field, has its own drawbacks and
limitations.

Fixes: fcc1d0d ("genirq: Add interrupt redirection infrastructure")
Reported-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Radu Rendec <rrendec@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Link: https://patch.msgid.link/20260112211402.2927336-1-rrendec@redhat.com
Closes: https://lore.kernel.org/all/44509520-f29b-4b8a-8986-5eae3e022eb7@nvidia.com/

Author: rrendec

kernel/irq/chip.c

@@ -1493,6 +1493,8 @@ int irq_chip_redirect_set_affinity(struct irq_data *data, const struct cpumask *
 	struct irq_redirect *redir = &irq_data_to_desc(data)->redirect;
 
 	WRITE_ONCE(redir->target_cpu, cpumask_first(dest));
+	irq_data_update_effective_affinity(data, dest);
+
 	return IRQ_SET_MASK_OK;
 }
 EXPORT_SYMBOL_GPL(irq_chip_redirect_set_affinity);