atomics: add volatile_read/volatile_set

Commit Message

Paolo Bonzini July 18, 2016, 5:22 p.m. UTC

On 18/07/2016 19:17, Sergey Fedorov wrote:
> On 18/07/16 20:11, Paolo Bonzini wrote:
>>
>> On 18/07/2016 19:07, Sergey Fedorov wrote:
>>> On 18/07/16 20:00, Paolo Bonzini wrote:
>>>> On 18/07/2016 18:57, Sergey Fedorov wrote:
>>>>> On 18/07/16 19:53, Paolo Bonzini wrote:
>>>>>> 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.
>>>>> But shouldn't they be atomic to avoid reading torn writes?
>>>> A torn write would probably fail to match anyway, but even if it doesn't
>>>> it is indistinguishable from a race, isn't it?
>>> I'm afraid, torn write can happen to be a false match against a wrong
>>> TB. In case of a race with atomic access we either get the right TB or
>>> an invalid one which couldn't match any valid CPU state. Probably, we
>>> have to make sure (and document this) that TB invalidation process
>>> cannot make a partially invalidated TB which can match any meaningful
>>> CPU state.
>> x86 is atomic (because flags are 32-bit); those that have cs_base==0 are
>> safe against torn writes too.  Only SPARC perhaps could use
>> "tb->cs_base|=1" instead in case 0xffffffff........ matches another TB.
> 
> That could really work but needs some comment, of course.

Yup.  A simpler possibility is this:



I'll send a fixup patch now, ack it and I'll send another pull request. :)

> BTW, what is
> the main point of such change? A bit more performance on some 32-bit hosts?

No, succeeding to compile on 32-bit hosts. :)

Patch

diff --git a/target-sparc/cpu.h b/target-sparc/cpu.h
index e327a35..3278d8a 100644
--- a/target-sparc/cpu.h
+++ b/target-sparc/cpu.h
@@ -753,14 +753,14 @@  static inline void cpu_get_invalid_tb_cpu_state(target_ulong *pc,
                                                 target_ulong *cs_base,
                                                 uint32_t *flags)
 {
-    *cs_base = -1; /* npc must be a multible of 4 */
+    *flags = TB_FLAG_MMU_MASK;
 }
 
 static inline bool cpu_tb_cpu_state_is_invalidated(target_ulong pc,
                                                    target_ulong cs_base,
                                                    uint32_t flags)
 {
-    return cs_base == -1;
+    return flags == TB_FLAG_MMU_MASK;
 }
 
 static inline bool tb_fpu_enabled(int tb_flags)