| Message ID | 1464179166-24137-1-git-send-email-pbonzini@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Wed, May 25, 2016 at 14:26:06 +0200, Paolo Bonzini wrote: > 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(-) > > diff --git a/docs/atomics.txt b/docs/atomics.txt > index 67a27ad..3f8b770 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 all of inc, dec, and, sub, and, or. Not clear whether the last 'and' is redundant or is being used as a conjunction. Either way it would be clearer to just remove it. Other than that: Reviewed-by: Emilio G. Cota <cota@braap.org> Emilio
On 25/05/2016 17:15, Emilio G. Cota wrote: >> > + >> > + In QEMU, the second kind does not exist. Currently Linux has >> > + atomic_fetch_or only; QEMU provides all of inc, dec, and, sub, and, or. > Not clear whether the last 'and' is redundant or is being used as > a conjunction. Either way it would be clearer to just remove it. The first "and" is "add". :) Thanks, Paolo
diff --git a/docs/atomics.txt b/docs/atomics.txt index 67a27ad..3f8b770 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 all of inc, dec, and, sub, and, or. - 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(-)