Re: [Qemu-devel] [PATCH] atomics: add volatile_read/volatile_set

[Paolo Bonzini <pbonzini@redhat.com>]

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 <pbonzini@redhat.com>
>> ---
>>  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
>