linux-kernel - Re: [PATCH 4/7] KVM: x86: Don't load/put vCPU when unloading its MMU during teardown

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Message-ID: <Z71tlzQJISk6PFAL@yzhao56-desk.sh.intel.com>
Date: Tue, 25 Feb 2025 15:13:27 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>,
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
	Huacai Chen <chenhuacai@...nel.org>, Madhavan Srinivasan
	<maddy@...ux.ibm.com>, Anup Patel <anup@...infault.org>, Paul Walmsley
	<paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou
	<aou@...s.berkeley.edu>, Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda
	<imbrenda@...ux.ibm.com>, Paolo Bonzini <pbonzini@...hat.com>,
	<linux-arm-kernel@...ts.infradead.org>, <kvmarm@...ts.linux.dev>,
	<kvm@...r.kernel.org>, <loongarch@...ts.linux.dev>,
	<linux-mips@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
	<kvm-riscv@...ts.infradead.org>, <linux-riscv@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, Aaron Lewis <aaronlewis@...gle.com>, "Jim
 Mattson" <jmattson@...gle.com>, Rick P Edgecombe
	<rick.p.edgecombe@...el.com>, Kai Huang <kai.huang@...el.com>, Isaku Yamahata
	<isaku.yamahata@...el.com>
Subject: Re: [PATCH 4/7] KVM: x86: Don't load/put vCPU when unloading its MMU
 during teardown

On Mon, Feb 24, 2025 at 03:55:39PM -0800, Sean Christopherson wrote:
> Don't load (and then put) a vCPU when unloading its MMU during VM
> destruction, as nothing in kvm_mmu_unload() accesses vCPU state beyond the
> root page/address of each MMU, i.e. can't possible need to run with the
> vCPU loaded.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 045c61cc7e54..9978ed4c0917 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12767,13 +12767,6 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  	return ret;
>  }
>  
> -static void kvm_unload_vcpu_mmu(struct kvm_vcpu *vcpu)
> -{
> -	vcpu_load(vcpu);
> -	kvm_mmu_unload(vcpu);
> -	vcpu_put(vcpu);
> -}
> -
>  static void kvm_unload_vcpu_mmus(struct kvm *kvm)
>  {
>  	unsigned long i;
> @@ -12781,7 +12774,7 @@ static void kvm_unload_vcpu_mmus(struct kvm *kvm)
>  
>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>  		kvm_clear_async_pf_completion_queue(vcpu);
> -		kvm_unload_vcpu_mmu(vcpu);
> +		kvm_mmu_unload(vcpu);
What about just dropping kvm_unload_vcpu_mmu() here?
kvm_mmu_unload() will be invoked again in kvm_mmu_destroy().

kvm_arch_vcpu_destroy() --> kvm_mmu_destroy() --> kvm_mmu_unload().

>  	}
>  }
>  
> -- 
> 2.48.1.658.g4767266eb4-goog
>