KVM ARM Archive on lore.kernel.org
 help / color / Atom feed
From: Guoheyi <guoheyi@huawei.com>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-kernel@vger.kernel.org,
	Will Deacon <will@kernel.org>,
Subject: Re: [PATCH] kvm/arm64: change gicv3_cpuif to static likely branch
Date: Sat, 30 Nov 2019 14:58:16 +0800
Message-ID: <496cb45d-c312-295c-18f2-633ec5acc976@huawei.com> (raw)
In-Reply-To: <86mucdzx45.wl-maz@kernel.org>

On 2019/11/30 14:39, Marc Zyngier wrote:
> 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).
Really? I'm surprised to know that... Sorry I didn't see such GICv2 
platforms in my work, so I made the wrong assumption.
I don't expect much performance improvement for GICv3 platforms. The 
precondition for this patch is that few platforms running KVM are using 
GICv2. If it is not right, please just ignore it.


> 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.

kvmarm mailing list

      reply index

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-30  3:14 Heyi Guo
2019-11-30  6:39 ` Marc Zyngier
2019-11-30  6:58   ` Guoheyi [this message]

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=496cb45d-c312-295c-18f2-633ec5acc976@huawei.com \
    --to=guoheyi@huawei.com \
    --cc=kvmarm@lists.cs.columbia.edu \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=maz@kernel.org \
    --cc=will@kernel.org \


* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

KVM ARM Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/kvmarm/0 kvmarm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 kvmarm kvmarm/ https://lore.kernel.org/kvmarm \
	public-inbox-index kvmarm

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone https://public-inbox.org/public-inbox.git