linux-kernel - Re: [PATCH v5 5/7] kvm, mem-hotplug: Reload L1's apic access page on migration when L2 is running.

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Date:	Thu, 11 Sep 2014 11:28:06 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Tang Chen <tangchen@...fujitsu.com>, gleb@...nel.org,
	mtosatti@...hat.com, nadav.amit@...il.com, jan.kiszka@....de
CC:	kvm@...r.kernel.org, laijs@...fujitsu.com,
	isimatu.yasuaki@...fujitsu.com, guz.fnst@...fujitsu.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 5/7] kvm, mem-hotplug: Reload L1's apic access page
 on migration when L2 is running.

Il 11/09/2014 07:38, Tang Chen ha scritto:
> This patch only handle "L1 and L2 vm share one apic access page" situation.
> 
> When L1 vm is running, if the shared apic access page is migrated, mmu_notifier will
> request all vcpus to exit to L0, and reload apic access page physical address for
> all the vcpus' vmcs (which is done by patch 5/6). And when it enters L2 vm, L2's vmcs
> will be updated in prepare_vmcs02() called by nested_vm_run(). So we need to do
> nothing.
> 
> When L2 vm is running, if the shared apic access page is migrated, mmu_notifier will
> request all vcpus to exit to L0, and reload apic access page physical address for
> all L2 vmcs. And this patch requests apic access page reload in L2->L1 vmexit.
> 
> Signed-off-by: Tang Chen <tangchen@...fujitsu.com>
> ---
>  arch/x86/kvm/vmx.c  | 7 +++++++
>  virt/kvm/kvm_main.c | 1 +
>  2 files changed, 8 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index da6d55d..e7704b2 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -8796,6 +8796,13 @@ static void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 exit_reason,
>  	}
>  
>  	/*
> +	 * Do not call kvm_reload_apic_access_page() because we are now
> +	 * running, mmu_notifier will force to reload the page's hpa for L2
> +	 * vmcs. Need to reload it for L1 before entering L1.
> +	 */
> +	kvm_reload_apic_access_page(vcpu->kvm);

That would kill performance for large L1 hosts.
vcpu_reload_apic_access_page should be enough (please rename it to
kvm_vcpu_reload_apic_access_page in patch 4, and export it).

Paolo

> +	/*
>  	 * Exiting from L2 to L1, we're now back to L1 which thinks it just
>  	 * finished a VMLAUNCH or VMRESUME instruction, so we need to set the
>  	 * success or failure flag accordingly.
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d8280de..784127e 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -214,6 +214,7 @@ void kvm_reload_apic_access_page(struct kvm *kvm)
>  {
>  	make_all_cpus_request(kvm, KVM_REQ_APIC_PAGE_RELOAD);
>  }
> +EXPORT_SYMBOL_GPL(kvm_reload_apic_access_page);
>  
>  int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
>  {
> 

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