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From: Richard Henderson <rth@twiddle.net>
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Date: Mon, 23 May 2016 09:53:00 -0700
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Subject: Re: [Qemu-devel] [PATCH 1/2] atomics: do not use __atomic
 primitives for RCU atomics
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To: "Emilio G. Cota" <cota@braap.org>, QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>
Cc: =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Sergey Fedorov <serge.fdrv@gmail.com>

On 05/21/2016 01:42 PM, Emilio G. Cota wrote:
> In the process, the atomic_rcu_read/set were converted to implement
> consume/release semantics, respectively. This is inefficient; for
> correctness and maximum performance we only need an smp_barrier_depends
> for reads, and an smp_wmb for writes. Fix it by using the original
> definition of these two primitives for all compilers.

For what host do you think this is inefficient?

In particular, what you've done is going to be less efficient for e.g. armv8, 
where the __atomic formulation is going to produce load-acquire and 
store-release instructions.  Whereas the separate barriers are going to produce 
two insns.

As for the common case of x86_64, what you're doing is going to make no 
difference at all.

So what are you trying to improve?


r~