KVM: x86: Extend KVM_SET_VCPU_EVENTS with selective updates

[safe/jmp/linux-2.6] / Documentation / local_ops.txt

diff --git a/Documentation/local_ops.txt b/Documentation/local_ops.txt
index b0aca07..300da4b 100644 (file)
--- a/Documentation/local_ops.txt
+++ b/Documentation/local_ops.txt
@@ -27,16 +27,16 @@ CPU which owns the data. Therefore, care must taken to make sure that only one
 CPU writes to the local_t data. This is done by using per cpu data and making
 sure that we modify it from within a preemption safe context. It is however
 permitted to read local_t data from any CPU : it will then appear to be written
-out of order wrt other memory writes on the owner CPU.
+out of order wrt other memory writes by the owner CPU.
 
 
 * Implementation for a given architecture
 
 It can be done by slightly modifying the standard atomic operations : only
 their UP variant must be kept. It typically means removing LOCK prefix (on
-i386 and x86_64) and any SMP sychronization barrier. If the architecture does
+i386 and x86_64) and any SMP synchronization barrier. If the architecture does
 not have a different behavior between SMP and UP, including asm-generic/local.h
-in your archtecture's local.h is sufficient.
+in your architecture's local.h is sufficient.
 
 The local_t type is defined as an opaque signed long by embedding an
 atomic_long_t inside a structure. This is made so a cast from this type to a
@@ -45,6 +45,29 @@ long fails. The definition looks like :
 typedef struct { atomic_long_t a; } local_t;
 
 
+* Rules to follow when using local atomic operations
+
+- Variables touched by local ops must be per cpu variables.
+- _Only_ the CPU owner of these variables must write to them.
+- This CPU can use local ops from any context (process, irq, softirq, nmi, ...)
+  to update its local_t variables.
+- Preemption (or interrupts) must be disabled when using local ops in
+  process context to   make sure the process won't be migrated to a
+  different CPU between getting the per-cpu variable and doing the
+  actual local op.
+- When using local ops in interrupt context, no special care must be
+  taken on a mainline kernel, since they will run on the local CPU with
+  preemption already disabled. I suggest, however, to explicitly
+  disable preemption anyway to make sure it will still work correctly on
+  -rt kernels.
+- Reading the local cpu variable will provide the current copy of the
+  variable.
+- Reads of these variables can be done from any CPU, because updates to
+  "long", aligned, variables are always atomic. Since no memory
+  synchronization is done by the writer CPU, an outdated copy of the
+  variable can be read when reading some _other_ cpu's variables.
+
+
 * How to use local atomic operations
 
 #include <linux/percpu.h>
@@ -126,7 +149,7 @@ static void do_test_timer(unsigned long data)
        int cpu;
 
        /* Increment the counters */
-       on_each_cpu(test_each, NULL, 0, 1);
+       on_each_cpu(test_each, NULL, 1);
        /* Read all the counters */
        printk("Counters read from CPU %d\n", smp_processor_id());
        for_each_online_cpu(cpu) {