Re: [Qemu-devel] [RFC v3 PATCH 14/14] target-i386: Generate fences for x86

[Paolo Bonzini]

On 21/06/2016 17:57, Richard Henderson wrote:
>>>
>>>                  || (prefixes & PREFIX_LOCK)) {
>>>                  goto illegal_op;
>>>              }
>>> +            tcg_gen_mb(TCG_MO_ST_ST | TCG_BAR_SC);
>>>              break;
>>>          case 0xe8 ... 0xef: /* lfence */
>>> +            tcg_gen_mb(TCG_MO_LD_LD | TCG_BAR_SC);
>>> +            break;
>>
>> These are unnecessary.  On the other hand, _each and every load_ must be
>> followed by a LD_LD | LD_ST barrier, and each and every store must be
>> preceded by a LD_ST | ST_ST barrier.
> 
> They're not unnecessary if we (1) add those barriers for normal loads
> and stores and (2) omit them from the non-temporal loads and stores.

When does TCG generate non-temporal loads and stores?

Paolo