Re: [RFC PATCH 0/4] Add support for ARMv8.6 TWED feature

From: Marc Zyngier <maz@kernel.org>
To: Jingyi Wang <wangjingyi11@huawei.com>
Cc: kvm@vger.kernel.org, suzuki.poulose@arm.com,
	catalin.marinas@arm.com, shameerali.kolothum.thodi@huawei.com,
	yezengruan@huawei.com, fanhenglong@huawei.com,
	james.morse@arm.com, julien.thierry.kdev@gmail.com,
	prime.zeng@hisilicon.com, wanghaibin.wang@huawei.com,
	will@kernel.org, kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH 0/4] Add support for ARMv8.6 TWED feature
Date: Tue, 29 Sep 2020 11:50:09 +0100	[thread overview]
Message-ID: <913250ae919fb9453feadd0527827d55@kernel.org> (raw)
In-Reply-To: <20200929091727.8692-1-wangjingyi11@huawei.com>

On 2020-09-29 10:17, Jingyi Wang wrote:
> TWE Delay is an optional feature in ARMv8.6 Extentions. There is a
> performance benefit in waiting for a period of time for an event to
> arrive before taking the trap as it is common that event will arrive
> “quite soon” after executing the WFE instruction.

Define "quite soon". Quantify "performance benefits". Which are the
workloads that actually benefit from this imitation of the x86 PLE?

I was opposed to this when the spec was drafted, and I still am given
that there is zero supporting evidence that it bring any gain over
immediate trapping in an oversubscribed environment (which is the only
case where it matters).

Thanks,

         M.

> 
> This series adds support for TWED feature and implements TWE delay
> value dynamic adjustment.
> 
> Thanks for Shameer's advice on this series. The function of this patch
> has been tested on TWED supported hardware and the performance of it is
> still on test, any advice will be welcomed.
> 
> Jingyi Wang (2):
>   KVM: arm64: Make use of TWED feature
>   KVM: arm64: Use dynamic TWE Delay value
> 
> Zengruan Ye (2):
>   arm64: cpufeature: TWED support detection
>   KVM: arm64: Add trace for TWED update
> 
>  arch/arm64/Kconfig                   | 10 +++++
>  arch/arm64/include/asm/cpucaps.h     |  3 +-
>  arch/arm64/include/asm/kvm_arm.h     |  5 +++
>  arch/arm64/include/asm/kvm_emulate.h | 38 ++++++++++++++++++
>  arch/arm64/include/asm/kvm_host.h    | 19 ++++++++-
>  arch/arm64/include/asm/virt.h        |  8 ++++
>  arch/arm64/kernel/cpufeature.c       | 12 ++++++
>  arch/arm64/kvm/arm.c                 | 58 ++++++++++++++++++++++++++++
>  arch/arm64/kvm/handle_exit.c         |  2 +
>  arch/arm64/kvm/trace_arm.h           | 21 ++++++++++
>  10 files changed, 174 insertions(+), 2 deletions(-)

-- 
Jazz is not dead. It just smells funny...

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