Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier

[Pranith Kumar]

On Mon, Jun 6, 2016 at 12:14 PM, Sergey Fedorov <address@hidden> wrote:
> On 06/06/16 18:58, Pranith Kumar wrote:
>> On Mon, Jun 6, 2016 at 11:49 AM, Sergey Fedorov <address@hidden> wrote:
>>> On 06/06/16 18:47, Pranith Kumar wrote:
>>>> On Mon, Jun 6, 2016 at 11:44 AM, Sergey Fedorov <address@hidden> wrote:
>>>>> On 03/06/16 21:27, Pranith Kumar wrote:
>>>>>> On Thu, Jun 2, 2016 at 5:18 PM, Richard Henderson <address@hidden> wrote:
>>>>>>> What if we have tcg_canonicalize_memop (or some such) split off the 
>>>>>>> barriers
>>>>>>> into separate opcodes.  E.g.
>>>>>>>
>>>>>>> MO_BAR_LD_B = 32        // prevent earlier loads from crossing current 
>>>>>>> op
>>>>>>> MO_BAR_ST_B = 64        // prevent earlier stores from crossing current 
>>>>>>> op
>>>>>>> MO_BAR_LD_A = 128       // prevent later loads from crossing current op
>>>>>>> MO_BAR_ST_A = 256       // prevent later stores from crossing current op
>>>>>>> MO_BAR_LDST_B = MO_BAR_LD_B | MO_BAR_ST_B
>>>>>>> MO_BAR_LDST_A = MO_BAR_LD_A | MO_BAR_ST_A
>>>>>>> MO_BAR_MASK = MO_BAR_LDST_B | MO_BAR_LDST_A
>>>>>>>
>>>>>>> // Match Sparc MEMBAR as the most flexible host.
>>>>>>> TCG_BAR_LD_LD = 1       // #LoadLoad barrier
>>>>>>> TCG_BAR_ST_LD = 2       // #StoreLoad barrier
>>>>>>> TCG_BAR_LD_ST = 4       // #LoadStore barrier
>>>>>>> TCG_BAR_ST_ST = 8       // #StoreStore barrier
>>>>>>> TCG_BAR_SYNC  = 64      // SEQ_CST barrier
>>>>>> I really like this format. I would also like to add to the frontend:
>>>>>>
>>>>> Actually, the acquire barrier is a combined load-load + load-store
>>>>> barrier; and the release barrier is a combo of load-store + store-store
>>>>> barriers.
>>>>>
>>>> All the above are two-way barriers. Acquire/Release are one-way
>>>> barriers. So we cannot combine the above to get acquire/release
>>>> semantics without being conservative.
>>> Do you mean *barriers* or *memory access* operations implying memory
>>> ordering?
>> I meant the latter. I know no arch which has acquire/release barriers.
>> Sorry for the confusion.
>
> So I am. By the way, what's the difference between sequential consistent
> *barrier* and a combination of all the other barriers?


If I read it correctly TCG_BAR_SYNC is equivalent to OR of all the
other four barriers. I am not sure if we can just construct SYNC like
this or if we need to define it explicitly though.

-- 
Pranith