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Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set |
Date: |
Mon, 18 Jul 2016 18:53:50 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 |
On 18/07/2016 18:52, Sergey Fedorov wrote:
> So how are we going to use them?
Instead of atomic_read/atomic_set when marking invalid TBs.
diff --git a/cpu-exec.c b/cpu-exec.c
index fd43de8..1275f3d 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -292,10 +292,10 @@ static inline TranslationBlock *tb_find(CPUState *cpu,
always be the same before a given translated block
is executed. */
cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
- tb = atomic_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
- if (unlikely(!tb || atomic_read(&tb->pc) != pc ||
- atomic_read(&tb->cs_base) != cs_base ||
- atomic_read(&tb->flags) != flags)) {
+ tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
+ if (unlikely(!tb || volatile_read(&tb->pc) != pc ||
+ volatile_read(&tb->cs_base) != cs_base ||
+ volatile_read(&tb->flags) != flags)) {
tb = tb_htable_lookup(cpu, pc, cs_base, flags);
if (!tb) {
diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
index 8f0afcd..35e963b 100644
--- a/include/exec/exec-all.h
+++ b/include/exec/exec-all.h
@@ -262,9 +262,9 @@ static inline void tb_mark_invalid(TranslationBlock *tb)
uint32_t flags = 0;
cpu_get_invalid_tb_cpu_state(&pc, &cs_base, &flags);
- atomic_set(&tb->pc, pc);
- atomic_set(&tb->cs_base, cs_base);
- atomic_set(&tb->flags, flags);
+ volatile_set(&tb->pc, pc);
+ volatile_set(&tb->cs_base, cs_base);
+ volatile_set(&tb->flags, flags);
}
static inline bool tb_is_invalid(TranslationBlock *tb)
Thanks,
Paolo
> 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, 2016/07/18
- Re: [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