diff options
| author | Emilio G. Cota | 2016-06-27 21:02:13 +0200 |
|---|---|---|
| committer | Richard Henderson | 2016-10-26 17:29:02 +0200 |
| commit | 1dd089d0eec060dcd8478735114d98421d414805 (patch) | |
| tree | 32f50b948a31b291aa092c3dd7c70a4e198d3bed /target-arm/helper-a64.h | |
| parent | target-arm: emulate SWP with atomic_xchg helper (diff) | |
| download | qemu-1dd089d0eec060dcd8478735114d98421d414805.tar.gz qemu-1dd089d0eec060dcd8478735114d98421d414805.tar.xz qemu-1dd089d0eec060dcd8478735114d98421d414805.zip | |
target-arm: emulate aarch64's LL/SC using cmpxchg helpers
Emulating LL/SC with cmpxchg is not correct, since it can
suffer from the ABA problem. Portable parallel code, however,
is written assuming only cmpxchg--and not LL/SC--is available.
This means that in practice emulating LL/SC with cmpxchg is
a viable alternative.
The appended emulates LL/SC pairs in aarch64 with cmpxchg helpers.
This works in both user and system mode. In usermode, it avoids
pausing all other CPUs to perform the LL/SC pair. The subsequent
performance and scalability improvement is significant, as the
plots below show. They plot the throughput of atomic_add-bench
compiled for ARM and executed on a 64-core x86 machine.
Hi-res plots: http://imgur.com/a/JVc8Y
atomic_add-bench: 1000000 ops/thread, [0,1] range
18 ++---------+----------+---------+----------+----------+----------+---++
+cmpxchg +-E--+ + + + + + |
16 ++master +-H--+ ++
|| |
14 ++ ++
| | |
12 ++| ++
| | |
10 ++++ ++
8 ++E ++
|+++ |
6 ++ | ++
| | |
4 ++ | ++
| | |
2 +H++E+--- ++
+ | +E++----+E+---+--+E+----++E+------+E+------+E++----+E+---+--+E|
0 ++H-H----H-+-----H----+---------+----------+----------+----------+---++
0 10 20 30 40 50 60
Number of threads
atomic_add-bench: 1000000 ops/thread, [0,2] range
18 ++---------+----------+---------+----------+----------+----------+---++
+cmpxchg +-E--+ + + + + + |
16 ++master +-H--+ ++
| | |
14 ++E ++
| | |
12 ++| ++
|+++ |
10 ++ | ++
8 ++ | ++
| | |
6 ++ | ++
| | |
4 ++ | ++
| +E+--- |
2 +H+ +E+-----+++ +++ +++ ---+E+-----+E+------+++
+++ + +E+---+--+E+----++E+------+E+--- ++++ +++ + +E|
0 ++H-H----H-+-----H----+---------+----------+----------+----------+---++
0 10 20 30 40 50 60
Number of threads
atomic_add-bench: 1000000 ops/thread, [0,128] range
70 ++---------+----------+---------+----------+----------+----------+---++
+cmpxchg +-E--+ + + + + + |
60 ++master +-H--+ +++ ---+E+-----+E+------+E+
| +E+------E-------+E+--- |
| --- +++ |
50 ++ +++--- ++
| -+E+ |
40 ++ +++---- ++
| E- |
| --| |
30 ++ -- +++ ++
| +E+ |
20 ++E+ ++
|E+ |
| |
10 ++ ++
+ + + + + + + |
0 +HH-H----H-+-----H----+---------+----------+----------+----------+---++
0 10 20 30 40 50 60
Number of threads
atomic_add-bench: 1000000 ops/thread, [0,1024] range
160 ++---------+---------+----------+---------+----------+----------+---++
+cmpxchg +-E--+ + + + + + |
140 ++master +-H--+ +++ +++
| -+E+-----+E+-------E|
120 ++ +++ ---- +++
| +++ ----E-- |
100 ++ --E--- +++ ++
| +++ ---- +++ |
80 ++ --E-- ++
| ---- +++ |
| -+E+ |
60 ++ ---- +++ ++
| +E+- |
40 ++ -- ++
| +E+ |
20 +EE+ ++
+++ + + + + + + |
0 +HH-H---H--+-----H---+----------+---------+----------+----------+---++
0 10 20 30 40 50 60
Number of threads
[rth: Rearrange 128-bit cmpxchg helper. Enforce alignment on LL.]
Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1467054136-10430-28-git-send-email-cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Diffstat (limited to 'target-arm/helper-a64.h')
| -rw-r--r-- | target-arm/helper-a64.h | 2 |
1 files changed, 2 insertions, 0 deletions
diff --git a/target-arm/helper-a64.h b/target-arm/helper-a64.h index 1d3d10fffb..dd32000e63 100644 --- a/target-arm/helper-a64.h +++ b/target-arm/helper-a64.h @@ -46,3 +46,5 @@ DEF_HELPER_FLAGS_2(frecpx_f32, TCG_CALL_NO_RWG, f32, f32, ptr) DEF_HELPER_FLAGS_2(fcvtx_f64_to_f32, TCG_CALL_NO_RWG, f32, f64, env) DEF_HELPER_FLAGS_3(crc32_64, TCG_CALL_NO_RWG_SE, i64, i64, i64, i32) DEF_HELPER_FLAGS_3(crc32c_64, TCG_CALL_NO_RWG_SE, i64, i64, i64, i32) +DEF_HELPER_FLAGS_4(paired_cmpxchg64_le, TCG_CALL_NO_WG, i64, env, i64, i64, i64) +DEF_HELPER_FLAGS_4(paired_cmpxchg64_be, TCG_CALL_NO_WG, i64, env, i64, i64, i64) |