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Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set
From: |
Sergey Fedorov |
Subject: |
Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set |
Date: |
Mon, 18 Jul 2016 19:52:08 +0300 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0 |
So how are we going to use them?
Thanks,
Sergey
On 18/07/16 17:17, Paolo Bonzini wrote:
> 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
- [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 <=
- 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
- Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set, Sergey Fedorov, 2016/07/18