Re: [PATCH 2/2] KVM: Call kvm_arch_vcpu_blocking early into the blocking sequence

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Marc Zyngier" <maz@kernel.org>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Julien Thierry" <julien.thierry.kdev@gmail.com>,
	"Suzuki K Poulose" <suzuki.poulose@arm.com>,
	"James Morse" <james.morse@arm.com>,
	"Joerg Roedel" <joro@8bytes.org>,
	"Suravee Suthikulpanit" <Suravee.Suthikulpanit@amd.com>,
	Tangnianyao <tangnianyao@huawei.com>
Cc: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org
Subject: Re: [PATCH 2/2] KVM: Call kvm_arch_vcpu_blocking early into the blocking sequence
Date: Fri, 2 Aug 2019 12:46:33 +0200	[thread overview]
Message-ID: <a7250b9a-60e9-b04c-5515-e506aea46515@redhat.com> (raw)
In-Reply-To: <20190802103709.70148-3-maz@kernel.org>

On 02/08/19 12:37, Marc Zyngier wrote:
> When a vpcu is about to block by calling kvm_vcpu_block, we call
> back into the arch code to allow any form of synchronization that
> may be required at this point (SVN stops the AVIC, ARM synchronises
> the VMCR and enables GICv4 doorbells). But this synchronization
> comes in quite late, as we've potentially waited for halt_poll_ns
> to expire.
> 
> Instead, let's move kvm_arch_vcpu_blocking() to the beginning of
> kvm_vcpu_block(), which on ARM has several benefits:
> 
> - VMCR gets synchronised early, meaning that any interrupt delivered
>   during the polling window will be evaluated with the correct guest
>   PMR
> - GICv4 doorbells are enabled, which means that any guest interrupt
>   directly injected during that window will be immediately recognised
> 
> Tang Nianyao ran some tests on a GICv4 machine to evaluate such
> change, and reported up to a 10% improvement for netperf:
> 
> <quote>
> 	netperf result:
> 	D06 as server, intel 8180 server as client
> 	with change:
> 	package 512 bytes - 5500 Mbits/s
> 	package 64 bytes - 760 Mbits/s
> 	without change:
> 	package 512 bytes - 5000 Mbits/s
> 	package 64 bytes - 710 Mbits/s
> </quote>
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  virt/kvm/kvm_main.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 887f3b0c2b60..90d429c703cb 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2322,6 +2322,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  	bool waited = false;
>  	u64 block_ns;
>  
> +	kvm_arch_vcpu_blocking(vcpu);
> +
>  	start = cur = ktime_get();
>  	if (vcpu->halt_poll_ns && !kvm_arch_no_poll(vcpu)) {
>  		ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
> @@ -2342,8 +2344,6 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  		} while (single_task_running() && ktime_before(cur, stop));
>  	}
>  
> -	kvm_arch_vcpu_blocking(vcpu);
> -
>  	for (;;) {
>  		prepare_to_swait_exclusive(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
>  
> @@ -2356,9 +2356,8 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>  
>  	finish_swait(&vcpu->wq, &wait);
>  	cur = ktime_get();
> -
> -	kvm_arch_vcpu_unblocking(vcpu);
>  out:
> +	kvm_arch_vcpu_unblocking(vcpu);
>  	block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
>  
>  	if (!vcpu_valid_wakeup(vcpu))
> 

Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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