[PATCH] x86: Add portable getcpu call

For NUMA optimization and some other algorithms it is useful to have a fast
to get the current CPU and node numbers in user space.

x86-64 added a fast way to do this in a vsyscall. This adds a generic
syscall for other architectures to make it a generic portable facility.

I expect some of them will also implement it as a faster vsyscall.

The cache is an optimization for the x86-64 vsyscall optimization. Since
what the syscall returns is an approximation anyways and user space
often wants very fast results it can be cached for some time.  The norma
methods to get this information in user space are relatively slow

The vsyscall is in a better position to manage the cache because it has direct
access to a fast time stamp (jiffies). For the generic syscall optimization
it doesn't help much, but enforce a valid argument to keep programs
portable

I only added an i386 syscall entry for now. Other architectures can follow
as needed.

AK: Also added some cleanups from Andrew Morton

Signed-off-by: Andi Kleen <ak@suse.de>

kernel/sys.c

@@ -28,6 +28,7 @@
 #include <linux/tty.h>
 #include <linux/signal.h>
 #include <linux/cn_proc.h>
+#include <linux/getcpu.h>
 
 #include <linux/compat.h>
 #include <linux/syscalls.h>
@@ -2062,3 +2063,33 @@ asmlinkage long sys_prctl(int option, unsigned long arg2, unsigned long arg3,
 	}
 	return error;
 }
+
+asmlinkage long sys_getcpu(unsigned __user *cpup, unsigned __user *nodep,
+	   		   struct getcpu_cache __user *cache)
+{
+	int err = 0;
+	int cpu = raw_smp_processor_id();
+	if (cpup)
+		err |= put_user(cpu, cpup);
+	if (nodep)
+		err |= put_user(cpu_to_node(cpu), nodep);
+	if (cache) {
+		/*
+		 * The cache is not needed for this implementation,
+		 * but make sure user programs pass something
+		 * valid. vsyscall implementations can instead make
+		 * good use of the cache. Only use t0 and t1 because
+		 * these are available in both 32bit and 64bit ABI (no
+		 * need for a compat_getcpu). 32bit has enough
+		 * padding
+		 */
+		unsigned long t0, t1;
+		get_user(t0, &cache->t0);
+		get_user(t1, &cache->t1);
+		t0++;
+		t1++;
+		put_user(t0, &cache->t0);
+		put_user(t1, &cache->t1);
+	}
+	return err ? -EFAULT : 0;
+}