linux-kernel - Re: [PATCH v2 01/31] KVM: x86: hyper-v: Resurrect dedicated KVM_REQ_HV_TLB

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Date:   Thu, 7 Apr 2022 18:02:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/31] KVM: x86: hyper-v: Resurrect dedicated
 KVM_REQ_HV_TLB_FLUSH flag

On Thu, Apr 07, 2022, Vitaly Kuznetsov wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e9647614dc8c..3c54f6804b7b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3341,7 +3341,12 @@ void kvm_service_local_tlb_flush_requests(struct kvm_vcpu *vcpu)
>  	if (kvm_check_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu))
>  		kvm_vcpu_flush_tlb_current(vcpu);
>  
> -	if (kvm_check_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu))
> +	if (kvm_check_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu)) {
> +		kvm_vcpu_flush_tlb_guest(vcpu);
> +		kvm_clear_request(KVM_REQ_HV_TLB_FLUSH, vcpu);
> +	}
> +
> +	if (kvm_check_request(KVM_REQ_HV_TLB_FLUSH, vcpu))
>  		kvm_vcpu_flush_tlb_guest(vcpu);

It'd be slightly more performant to do:

	if (kvm_check_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu)) {
		kvm_vcpu_flush_tlb_guest(vcpu);
		kvm_clear_request(KVM_REQ_HV_TLB_FLUSH, vcpu);
	} else if (kvm_check_request(KVM_REQ_HV_TLB_FLUSH, vcpu)) {
		kvm_hv_vcpu_flush_tlb(vcpu);
	}

And then when the code becomes


	if (kvm_check_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu)) {
		kvm_vcpu_flush_tlb_guest(vcpu);
		if (kvm_check_request(KVM_REQ_HV_TLB_FLUSH, vcpu))
			kvm_hv_vcpu_empty_flush_tlb(vcpu);
	} else if (kvm_check_request(KVM_REQ_HV_TLB_FLUSH, vcpu)) {
		kvm_hv_vcpu_flush_tlb(vcpu);
	}

the elif will help unsuspecting readers see that the HV_TLB_FLUSH request is
cleared by kvm_check_request() in the TLB_FLUSH_GUEST path.

The elif could result in having to bail from VM-Entry if the request becomes
pending after the check/clear inside TLB_FLUSH_GUEST, but that should be a very
rare case.

>  }
>  EXPORT_SYMBOL_GPL(kvm_service_local_tlb_flush_requests);
> -- 
> 2.35.1
>