Commit a4a0e4b2 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

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: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 15487aa1

docs/atomics.txt

+15 −5
Original line number Diff line number Diff line
@@ -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.





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