From a4a0e4b258b2034559a546927794f477025d55d6 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Wed, 25 May 2016 14:23:27 +0200 Subject: [PATCH] docs/atomics: update comparison with Linux 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 --- docs/atomics.txt | 20 +++++++++++++++----- 1 file 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 -- 2.11.4.GIT