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

From: Peter Maydell
Subject: Re: [Qemu-devel] [RFC v2 PATCH 01/13] Introduce TCGOpcode for memory barrier
Date: Fri, 3 Jun 2016 21:43:02 +0100

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).

-- PMM

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