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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Fri, 03 Feb 2017 11:30:12 +0000
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Subject: Re: [Qemu-devel] [PATCH v9 21/25] target-arm: don't generate
 WFE/YIELD calls for MTTCG
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: MTTCG Devel <mttcg@listserver.greensocs.com>, QEMU Developers <qemu-devel@nongnu.org>, KONRAD =?utf-8?B?RnLDqWTDqXJpYw==?= <fred.konrad@greensocs.com>, Alvise Rigo <a.rigo@virtualopensystems.com>, "Emilio G. Cota" <cota@braap.org>, Pranith Kumar <bobby.prani@gmail.com>, Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>, Mark Burton <mark.burton@greensocs.com>, Paolo Bonzini <pbonzini@redhat.com>, Jan Kiszka <jan.kiszka@siemens.com>, Fedorov Sergey <serge.fdrv@gmail.com>, Richard Henderson <rth@twiddle.net>, Bamvor Zhang Jian <bamvor.zhangjian@linaro.org>, "open list:ARM" <qemu-arm@nongnu.org>


Peter Maydell <peter.maydell@linaro.org> writes:

> On 1 February 2017 at 15:05, Alex Bennée <alex.bennee@linaro.org> wrote:
>> The WFE and YIELD instructions are really only hints and in TCG's case
>> they were useful to move the scheduling on from one vCPU to the next. In
>> the parallel context (MTTCG) this just causes an unnecessary cpu_exit
>> and contention of the BQL.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> Reviewed-by: Richard Henderson <rth@twiddle.net>
>> ---
>>  target/arm/op_helper.c     |  7 +++++++
>>  target/arm/translate-a64.c |  8 ++++++--
>>  target/arm/translate.c     | 20 ++++++++++++++++----
>>  3 files changed, 29 insertions(+), 6 deletions(-)
>
> Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
>
> though I'm not much of a fan of yet another global variable :-(

It's unfortunate but I think justified in the case. parallel_cpus is a
global state that controls the emission of barriers and generation of
atomics. It does sometime get turned off for the EXCP_ATOMIC processing
(but only in a temporary exclusive region).

>
> thanks
> -- PMM


--
Alex Bennée