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

[Alex Bennée]

Sergey Fedorov <address@hidden> writes:

> On 03/06/16 21:30, Pranith Kumar wrote:
>> On Thu, Jun 2, 2016 at 9:08 PM, Richard Henderson <address@hidden> wrote:
>>> On 06/02/2016 02:37 PM, Sergey Fedorov wrote:
>>>>
>>>> It would give us three TCG operations for each memory operation instead
>>>> of one. But then we might like to combine these barrier operations back
>>>> with memory operations in each backend. If we propagate memory ordering
>>>> semantics up to the backend, it can decide itself what instructions are
>>>> best to generate.
>>>
>>> A strongly ordered target would generally only set BEFORE bits or AFTER
>>> bits, but not both (and I suggest we canonicalize on AFTER for all such
>>> targets). Thus a strongly ordered target would produce only 2 opcodes per
>>> memory op.
>>>
>>> I supplied both to make it easier to handle a weakly ordered target with
>>> acquire/release bits.
>>>
>>> I would *not* combine the barrier operations back with memory operations in
>>> the backend.  Only armv8 and ia64 can do that, and given the optimization
>>> level at which we generate code, I doubt it would really make much
>>> difference above separate barriers.
>>>
>> On armv8, using load_acquire/store_release instructions makes a
>> significant difference in performance when compared to plain
>> dmb+memory instruction sequence. So I would really like to keep the
>> option of generating acq/rel instructions(by combining barrier and
>> memory or some other way) open.
>
> I'm not so sure about acq/rel flags. Is there any architecture which has
> explicit acq/rel barriers? I suppose acq/rel memory access instructions
> are always load-link and store-conditional and thus rely on exclusive
> memory monitor to support that "conditional" behaviour.

Nope, you can have acq/rel memory operations without exclusive
operations (see ARMv8 ldar and stlr). The exclusive operations also have
ordered and non-ordered variants (ldxr, strx).

> To emulate this
> behaviour we need something more special like "Slow-path for atomic
> instruction translation" [1].
>
> [1] http://thread.gmane.org/gmane.comp.emulators.qemu/407419
>
> Kind regards,
> Sergey


--
Alex Bennée