linux-kernel - Re: [PATCH] KVM/VMX: Check ept_pointer before flushing ept tlb

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Date:   Tue, 06 Nov 2018 16:50:26 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     ltykernel@...il.com
Cc:     Lan Tianyu <Tianyu.Lan@...rosoft.com>, pbonzini@...hat.com,
        rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, michael.h.kelley@...rosoft.com,
        kys@...rosoft.com
Subject: Re: [PATCH] KVM/VMX: Check ept_pointer before flushing ept tlb

ltykernel@...il.com writes:

> From: Lan Tianyu <Tianyu.Lan@...rosoft.com>
>
> This patch is to initialize ept_pointer to INVALID_PAGE and check it
> before flushing ept tlb. If ept_pointer is invalidated, bypass the flush
> request.
>
> Signed-off-by: Lan Tianyu <Tianyu.Lan@...rosoft.com>
> ---
>  arch/x86/kvm/vmx.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 4555077d69ce..edbc96cb990a 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -1580,14 +1580,22 @@ static int vmx_hv_remote_flush_tlb(struct kvm *kvm)
>  	/*
>  	 * FLUSH_GUEST_PHYSICAL_ADDRESS_SPACE hypercall needs the address of the
>  	 * base of EPT PML4 table, strip off EPT configuration information.
> +	 * If ept_pointer is invalid pointer, bypass the flush request.
>  	 */
>  	if (to_kvm_vmx(kvm)->ept_pointers_match != EPT_POINTERS_MATCH) {
> -		kvm_for_each_vcpu(i, vcpu, kvm)
> +		kvm_for_each_vcpu(i, vcpu, kvm) {
> +			if (!VALID_PAGE(to_vmx(vcpu)->ept_pointer))
> +				return 0;
> +

To be honest I fail to understand the reason behind the patch: instead
of doing one unneeded flush request with ept_pointer==0 (after vCPU is
initialized) we now do the check every time. Could you please elaborate
on why this is needed?


>  			ret |= hyperv_flush_guest_mapping(
> -				to_vmx(kvm_get_vcpu(kvm, i))->ept_pointer & PAGE_MASK);
> +				to_vmx(vcpu)->ept_pointer & PAGE_MASK);

I would use a local variable for 'to_vmx(vcpu)->ept_pointer' or even
'to_vmx(vcpu)->ept_pointer & PAGE_MASK' and use it in VALID_PAGE() - as
lower bits are unrelated;


> +		}
>  	} else {
> +		if (!VALID_PAGE(to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer))
> +			return 0;

Ditto.

> +
>  		ret = hyperv_flush_guest_mapping(
> -				to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer & PAGE_MASK);
> +			to_vmx(kvm_get_vcpu(kvm, 0))->ept_pointer & PAGE_MASK);

This doesn't belong to this patch.

>  	}
>  
>  	spin_unlock(&to_kvm_vmx(kvm)->ept_pointer_lock);
> @@ -11568,6 +11576,8 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
>  	vmx->pi_desc.nv = POSTED_INTR_VECTOR;
>  	vmx->pi_desc.sn = 1;
>  
> +	vmx->ept_pointer = INVALID_PAGE;
> +
>  	return &vmx->vcpu;
>  
>  free_vmcs:

-- 
Vitaly