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From: Marc Zyngier <maz@kernel.org>
To: Heyi Guo <guoheyi@huawei.com>
Cc: linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH] kvm/arm64: change gicv3_cpuif to static likely branch
Date: Sat, 30 Nov 2019 06:39:22 +0000	[thread overview]
Message-ID: <86mucdzx45.wl-maz@kernel.org> (raw)
In-Reply-To: <20191130031443.41696-1-guoheyi@huawei.com>

On Sat, 30 Nov 2019 03:14:43 +0000,
Heyi Guo <guoheyi@huawei.com> wrote:
> 
> Platforms running hypervisor nowadays are normally powerful servers
> which at least support GICv3, so it should be better to switch
> kvm_vgic_global_state.gicv3_cpuif to static likely branch, which can
> reduce two "b" instructions to a single "nop" for GICv3 branches.
> 
> We don't update arm32 specific code for they may still only have
> GICv2.

There is a number of disputable statements here.

Out of the fairly large zoo of arm64 systems I have access to, 75% of
them are based on GICv2, so they are still the overwhelming majority.
Yes, they all run KVM (otherwise I would ignore them).

Furthermore, I would expect that "powerful servers" are perfectly
capable to execute a couple of branches without breaking a sweat.

Finally, you don't provide any number supporting that:

- GICv3 systems see a performance improvement across the large variety
  of CPU implementations
- GICv2 systems don't see a performance regression

Once you provide such numbers, I'll reevaluate my position. Until
then, I'm not considering this kind of change.

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.
_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

WARNING: multiple messages have this Message-ID (diff)
From: Marc Zyngier <maz@kernel.org>
To: Heyi Guo <guoheyi@huawei.com>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>,
	linux-kernel@vger.kernel.org, James Morse <james.morse@arm.com>,
	linux-arm-kernel@lists.infradead.org, wanghaibin.wang@huawei.com,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	Julien Thierry <julien.thierry.kdev@gmail.com>
Subject: Re: [PATCH] kvm/arm64: change gicv3_cpuif to static likely branch
Date: Sat, 30 Nov 2019 06:39:22 +0000	[thread overview]
Message-ID: <86mucdzx45.wl-maz@kernel.org> (raw)
In-Reply-To: <20191130031443.41696-1-guoheyi@huawei.com>

On Sat, 30 Nov 2019 03:14:43 +0000,
Heyi Guo <guoheyi@huawei.com> wrote:
> 
> Platforms running hypervisor nowadays are normally powerful servers
> which at least support GICv3, so it should be better to switch
> kvm_vgic_global_state.gicv3_cpuif to static likely branch, which can
> reduce two "b" instructions to a single "nop" for GICv3 branches.
> 
> We don't update arm32 specific code for they may still only have
> GICv2.

There is a number of disputable statements here.

Out of the fairly large zoo of arm64 systems I have access to, 75% of
them are based on GICv2, so they are still the overwhelming majority.
Yes, they all run KVM (otherwise I would ignore them).

Furthermore, I would expect that "powerful servers" are perfectly
capable to execute a couple of branches without breaking a sweat.

Finally, you don't provide any number supporting that:

- GICv3 systems see a performance improvement across the large variety
  of CPU implementations
- GICv2 systems don't see a performance regression

Once you provide such numbers, I'll reevaluate my position. Until
then, I'm not considering this kind of change.

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

WARNING: multiple messages have this Message-ID (diff)
From: Marc Zyngier <maz@kernel.org>
To: Heyi Guo <guoheyi@huawei.com>
Cc: <linux-arm-kernel@lists.infradead.org>,
	<kvmarm@lists.cs.columbia.edu>, <linux-kernel@vger.kernel.org>,
	<wanghaibin.wang@huawei.com>, James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH] kvm/arm64: change gicv3_cpuif to static likely branch
Date: Sat, 30 Nov 2019 06:39:22 +0000	[thread overview]
Message-ID: <86mucdzx45.wl-maz@kernel.org> (raw)
In-Reply-To: <20191130031443.41696-1-guoheyi@huawei.com>

On Sat, 30 Nov 2019 03:14:43 +0000,
Heyi Guo <guoheyi@huawei.com> wrote:
> 
> Platforms running hypervisor nowadays are normally powerful servers
> which at least support GICv3, so it should be better to switch
> kvm_vgic_global_state.gicv3_cpuif to static likely branch, which can
> reduce two "b" instructions to a single "nop" for GICv3 branches.
> 
> We don't update arm32 specific code for they may still only have
> GICv2.

There is a number of disputable statements here.

Out of the fairly large zoo of arm64 systems I have access to, 75% of
them are based on GICv2, so they are still the overwhelming majority.
Yes, they all run KVM (otherwise I would ignore them).

Furthermore, I would expect that "powerful servers" are perfectly
capable to execute a couple of branches without breaking a sweat.

Finally, you don't provide any number supporting that:

- GICv3 systems see a performance improvement across the large variety
  of CPU implementations
- GICv2 systems don't see a performance regression

Once you provide such numbers, I'll reevaluate my position. Until
then, I'm not considering this kind of change.

Thanks,

	M.

-- 
Jazz is not dead, it just smells funny.

  reply	other threads:[~2019-11-30  6:39 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-30  3:14 [PATCH] kvm/arm64: change gicv3_cpuif to static likely branch Heyi Guo
2019-11-30  3:14 ` Heyi Guo
2019-11-30  3:14 ` Heyi Guo
2019-11-30  6:39 ` Marc Zyngier [this message]
2019-11-30  6:39   ` Marc Zyngier
2019-11-30  6:39   ` Marc Zyngier
2019-11-30  6:58   ` Guoheyi
2019-11-30  6:58     ` Guoheyi
2019-11-30  6:58     ` Guoheyi

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