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.
next prev parent reply other threads:[~2019-11-30 6:39 UTC|newest]
Thread overview: 3+ 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 6:39 ` Marc Zyngier [this message]
2019-11-30 6:58 ` Guoheyi
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