@@ -340,17 +340,27 @@ and memory barriers, and the equivalents in QEMU:
properly aligned.
No barriers are implied by atomic_read/set in either Linux or QEMU.
-- most atomic read-modify-write operations in Linux return void;
- in QEMU, all of them return the old value of the variable.
+- atomic read-modify-write operations in Linux are of three kinds:
+
+ atomic_OP returns void
+ atomic_OP_return returns new value of the variable
+ atomic_fetch_OP returns the old value of the variable
+ atomic_cmpxchg returns the old value of the variable
+
+ In QEMU, the second kind does not exist. Currently Linux has
+ atomic_fetch_or only. QEMU provides and, or, inc, dec, add, sub.
- different atomic read-modify-write operations in Linux imply
a different set of memory barriers; in QEMU, all of them enforce
sequential consistency, which means they imply full memory barriers
before and after the operation.
-- Linux does not have an equivalent of atomic_mb_read() and
- atomic_mb_set(). In particular, note that set_mb() is a little
- weaker than atomic_mb_set().
+- Linux does not have an equivalent of atomic_mb_set(). In particular,
+ note that smp_store_mb() is a little weaker than atomic_mb_set().
+ atomic_mb_read() compiles to the same instructions as Linux's
+ smp_load_acquire(), but this should be treated as an implementation
+ detail. If required, QEMU might later add atomic_load_acquire() and
+ atomic_store_release() macros.
SOURCES
Over time, some differences between QEMU and Linux atomics are getting smoothed. In particular, Linux grew atomic_fetch_or (and in general the differences regarding RMW operations were not described accurately) and smp_load_acquire/smp_store_release. Also, set_mb was renamed to smp_store_mb(). Include these changes in the documentation. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> --- docs/atomics.txt | 20 +++++++++++++++----- 1 file changed, 15 insertions(+), 5 deletions(-)