linux-kernel - Re: [PATCH] kvm/arm64: change gicv3

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Message-ID: <86mucdzx45.wl-maz@kernel.org>
Date:   Sat, 30 Nov 2019 06:39:22 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Heyi Guo <guoheyi@...wei.com>
Cc:     <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <linux-kernel@...r.kernel.org>,
        <wanghaibin.wang@...wei.com>, James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] kvm/arm64: change gicv3_cpuif to static likely branch

On Sat, 30 Nov 2019 03:14:43 +0000,
Heyi Guo <guoheyi@...wei.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.