[PATCH] docs: kernel-parameters: document scope of irqaffinity= parameter

[Aaron Tomlin <atomlin@atomlin.com>]

System administrators frequently use the "irqaffinity=" boot parameter
in conjunction with CPU isolation to build deterministic, latency-free
environments. However, there is a widespread misconception that
"irqaffinity=" acts as a global, absolute override for all hardware
interrupts.

In reality, "irqaffinity=" strictly populates the irq_default_affinity
mask. When the kernel allocates multiqueue vectors
(e.g., irq_create_affinity_masks()), it explicitly bypasses this default
mask for managed interrupts. Instead, it relies on dynamic spreading
algorithms to map queues to the available topology, effectively
overriding any default the administrator set via the command line.

This patch explicitly documents this limitation in kernel-parameters.txt
to set correct expectations and directs users to the appropriate
"isolcpus=" sub-parameters for managed interrupt isolation.

Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
---
 Documentation/admin-guide/kernel-parameters.txt | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9ed7c3ecd158..40ca92d8cf04 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2732,6 +2732,14 @@ Kernel parameters
 	irqaffinity=	[SMP] Set the default irq affinity mask
 			The argument is a cpu list, as described above.
 
+			Note: This parameter only sets the default affinity
+			for unmanaged interrupts (e.g., legacy single-queue
+			devices or unmanaged pre/post vectors). It is
+			explicitly ignored by managed interrupts, such as
+			those utilised by modern multiqueue storage
+			controllers. To isolate CPUs from managed
+			interrupts, see the "managed_irq".
+
 	irqchip.gicv2_force_probe=
 			[ARM,ARM64,EARLY]
 			Format: <bool>
-- 
2.51.0