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| -rw-r--r-- | docs/atomics.txt | 20 |
1 files changed, 15 insertions, 5 deletions
diff --git a/docs/atomics.txt b/docs/atomics.txt index 67a27ad70a..c95950b6c5 100644 --- a/docs/atomics.txt +++ b/docs/atomics.txt @@ -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 |