[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set
From: |
Paolo Bonzini |
Subject: |
[Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set |
Date: |
Mon, 18 Jul 2016 16:17:30 +0200 |
Signed-off-by: Paolo Bonzini <address@hidden>
---
docs/atomics.txt | 19 ++++++++++++++++---
include/qemu/atomic.h | 17 +++++++++++++++++
2 files changed, 33 insertions(+), 3 deletions(-)
diff --git a/docs/atomics.txt b/docs/atomics.txt
index c95950b..1f21d2e 100644
--- a/docs/atomics.txt
+++ b/docs/atomics.txt
@@ -123,6 +123,14 @@ to do so, because it tells readers which variables are
shared with
other threads, and which are local to the current thread or protected
by other, more mundane means.
+atomic_read() and atomic_set() only support accesses as large as a
+pointer. If you need to access variables larger than a pointer you
+can use volatile_read() and volatile_set(), but be careful: these always
+use volatile accesses, and 64-bit volatile accesses are not atomic on
+several 32-bit processors such as ARMv7. In other words, volatile_read
+and volatile_set only provide "safe register" semantics when applied to
+64-bit variables.
+
Memory barriers control the order of references to shared memory.
They come in four kinds:
@@ -335,11 +343,16 @@ and memory barriers, and the equivalents in QEMU:
Both semantics prevent the compiler from doing certain transformations;
the difference is that atomic accesses are guaranteed to be atomic,
while volatile accesses aren't. Thus, in the volatile case we just cross
- our fingers hoping that the compiler will generate atomic accesses,
- since we assume the variables passed are machine-word sized and
- properly aligned.
+ our fingers hoping that the compiler and processor will provide atomic
+ accesses, since we assume the variables passed are machine-word sized
+ and properly aligned.
+
No barriers are implied by atomic_read/set in either Linux or QEMU.
+- volatile_read and volatile_set are equivalent to ACCESS_ONCE in Linux.
+ No barriers are implied by volatile_read/set in QEMU, nor by
+ ACCESS_ONCE in Linux.
+
- atomic read-modify-write operations in Linux are of three kinds:
atomic_OP returns void
diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 7e13fca..8409bdb 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -18,6 +18,12 @@
/* Compiler barrier */
#define barrier() ({ asm volatile("" ::: "memory"); (void)0; })
+/* These will only be atomic if the processor does the fetch or store
+ * in a single issue memory operation
+ */
+#define volatile_read(ptr) (*(__typeof__(*ptr) volatile*) (ptr))
+#define volatile_set(ptr, i) ((*(__typeof__(*ptr) volatile*) (ptr)) = (i))
+
#ifdef __ATOMIC_RELAXED
/* For C11 atomic ops */
@@ -260,6 +266,17 @@
*/
#define atomic_read(ptr) (*(__typeof__(*ptr) volatile*) (ptr))
#define atomic_set(ptr, i) ((*(__typeof__(*ptr) volatile*) (ptr)) = (i))
+#define atomic_read(ptr) \
+ ({ \
+ QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
+ volatile_read(ptr); \
+ })
+
+#define atomic_set(ptr, i) do { \
+ QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
+ volatile_set(ptr, i); \
+} while(0)
+
/**
* atomic_rcu_read - reads a RCU-protected pointer to a local variable
--
2.7.4
- [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Sergey Fedorov, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Paolo Bonzini, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Sergey Fedorov, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Paolo Bonzini, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Sergey Fedorov, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Paolo Bonzini, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Sergey Fedorov, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Paolo Bonzini, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Sergey Fedorov, 2016/07/18
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Paolo Bonzini, 2016/07/18