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+IRQ affinity on IA64 platforms
+ 07.01.2002, Erich Focht <>
+By writing to /proc/irq/IRQ#/smp_affinity the interrupt routing can be
+controlled. The behavior on IA64 platforms is slightly different from
+that described in Documentation/IRQ-affinity.txt for i386 systems.
+Because of the usage of SAPIC mode and physical destination mode the
+IRQ target is one particular CPU and cannot be a mask of several
+CPUs. Only the first non-zero bit is taken into account.
+Usage examples:
+The target CPU has to be specified as a hexadecimal CPU mask. The
+first non-zero bit is the selected CPU. This format has been kept for
+compatibility reasons with i386.
+Set the delivery mode of interrupt 41 to fixed and route the
+interrupts to CPU #3 (logical CPU number) (2^3=0x08):
+ echo "8" >/proc/irq/41/smp_affinity
+Set the default route for IRQ number 41 to CPU 6 in lowest priority
+delivery mode (redirectable):
+ echo "r 40" >/proc/irq/41/smp_affinity
+The output of the command
+ cat /proc/irq/IRQ#/smp_affinity
+gives the target CPU mask for the specified interrupt vector. If the CPU
+mask is preceded by the character "r", the interrupt is redirectable
+(i.e. lowest priority mode routing is used), otherwise its route is
+Initialization and default behavior:
+If the platform features IRQ redirection (info provided by SAL) all
+IO-SAPIC interrupts are initialized with CPU#0 as their default target
+and the routing is the so called "lowest priority mode" (actually
+fixed SAPIC mode with hint). The XTP chipset registers are used as hints
+for the IRQ routing. Currently in Linux XTP registers can have three
+ - minimal for an idle task,
+ - normal if any other task runs,
+ - maximal if the CPU is going to be switched off.
+The IRQ is routed to the CPU with lowest XTP register value, the
+search begins at the default CPU. Therefore most of the interrupts
+will be handled by CPU #0.
+If the platform doesn't feature interrupt redirection IOSAPIC fixed
+routing is used. The target CPUs are distributed in a round robin
+manner. IRQs will be routed only to the selected target CPUs. Check
+ cat /proc/interrupts
+On large (multi-node) systems it is recommended to route the IRQs to
+the node to which the corresponding device is connected.
+For systems like the NEC AzusA we get IRQ node-affinity for free. This
+is because usually the chipsets on each node redirect the interrupts
+only to their own CPUs (as they cannot see the XTP registers on the
+other nodes).