linux-kernel - Re: [PATCH 2/2] KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date

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Message-ID: <87r242547k.fsf@vitty.brq.redhat.com>
Date:   Fri, 27 Sep 2019 14:11:27 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reto Buerki <reet@...elabs.ch>
Subject: Re: [PATCH 2/2] KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Skip the VMWRITE to update GUEST_CR3 if CR3 is not available, i.e. has
> not been read from the VMCS since the last VM-Enter.  If vcpu->arch.cr3
> is stale, kvm_read_cr3(vcpu) will refresh vcpu->arch.cr3 from the VMCS,
> meaning KVM will do a VMREAD and then VMWRITE the value it just pulled
> from the VMCS.
>
> Note, this is a purely theoretical change, no instances of skipping
> the VMREAD+VMWRITE have been observed with this change.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index b530950a9c2b..6de09f60edf3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -3003,10 +3003,12 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>  
>  		if (is_guest_mode(vcpu))
>  			skip_cr3 = true;
> -		else if (enable_unrestricted_guest || is_paging(vcpu))
> -			guest_cr3 = kvm_read_cr3(vcpu);
> -		else
> +		else if (!enable_unrestricted_guest && !is_paging(vcpu))
>  			guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
> +		else if (test_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_avail))

Nit: with 'test_bit(,(ulong *)&vcpu->arch.regs_avail)' spreading more and
more I'd suggest creating an inline in kvm_cache_regs.h
(e.g. kvm_vcpu_reg_avail()).

> +			guest_cr3 = vcpu->arch.cr3;
> +		else
> +			skip_cr3 = true; /* vmcs01.GUEST_CR3 is up-to-date. */
>  		ept_load_pdptrs(vcpu);
>  	}

-- 
Vitaly