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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: Sat, 4 Jun 2016 00:33:30 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

On 03/06/16 23:43, Peter Maydell wrote:
> On 3 June 2016 at 20:49, Sergey Fedorov <address@hidden> wrote:
>> 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.
> This doesn't sound right (at least not for ARM). You can have
> load-acquire and store-release insns which aren't exclusives,
> you can have exclusives which aren't acq/rel, and you can
> have accesses which are both exclusives and acq/rel (and
> just to complete the set, obviously there are accesses which
> are neither). The exclusive semantics require the monitor, but
> acq/rel is just an ordering constraint (sort of like an implicit
> barrier, but not quite).

Thanks for clarifying this, Peter.


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