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

From: Sergey Fedorov
Subject: Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier
Date: Fri, 3 Jun 2016 22:49:01 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

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. 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,

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