linux-kernel - Re: [PATCH 15/15] KVM: nVMX: Always write vmcs.GUEST

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Date:   Thu, 18 Nov 2021 16:52:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 15/15] KVM: nVMX: Always write vmcs.GUEST_CR3 during
 nested VM-Exit

On 11/8/21 13:44, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> For VM-Enter, vmcs.GUEST_CR3 and vcpu->arch.cr3 are synced and it is
> better to mark VCPU_EXREG_CR3 available rather than dirty to reduce a
> redundant vmwrite(GUEST_CR3) in vmx_load_mmu_pgd().
> 
> But nested_vmx_load_cr3() is also served for VM-Exit which doesn't
> set vmcs.GUEST_CR3.
> 
> This patch moves writing to vmcs.GUEST_CR3 into nested_vmx_load_cr3()
> for both nested VM-Eneter/Exit and use kvm_register_mark_available().
> 
> This patch doesn't cause any extra writing to vmcs.GUEST_CR3 and if
> userspace is modifying CR3 with KVM_SET_SREGS later, the dirty info
> for VCPU_EXREG_CR3 would be set for next writing to vmcs.GUEST_CR3
> and no update will be lost.
> 
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
>   arch/x86/kvm/vmx/nested.c | 32 +++++++++++++++++++++-----------
>   1 file changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index ee5a68c2ea3a..4ddd4b1b0503 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -1133,8 +1133,28 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3,
>   	if (!nested_ept)
>   		kvm_mmu_new_pgd(vcpu, cr3);
>   
> +	/*
> +	 * Immediately write vmcs.GUEST_CR3 when changing vcpu->arch.cr3.
> +	 *
> +	 * VCPU_EXREG_CR3 is marked available rather than dirty because
> +	 * vcpu->arch.cr3 and vmcs.GUEST_CR3 are synced when enable_ept and
> +	 * vmcs.GUEST_CR3 is irrelevant to vcpu->arch.cr3 when !enable_ept.
> +	 *
> +	 * For VM-Enter case, it will be propagated to vmcs12 on nested
> +	 * VM-Exit, which can occur without actually running L2 and thus
> +	 * without hitting vmx_load_mmu_pgd(), e.g. if L1 is entering L2 with
> +	 * vmcs12.GUEST_ACTIVITYSTATE=HLT, in which case KVM will intercept
> +	 * the transition to HLT instead of running L2.
> +	 *
> +	 * For VM-Exit case, it is likely that vmcs.GUEST_CR3 == cr3 here, but
> +	 * L1 may set HOST_CR3 to a value other than its CR3 before VM-Entry,
> +	 * so we just update it unconditionally.
> +	 */
> +	if (enable_ept)
> +		vmcs_writel(GUEST_CR3, cr3);
> +
>   	vcpu->arch.cr3 = cr3;
> -	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
> +	kvm_register_mark_available(vcpu, VCPU_EXREG_CR3);
>   
>   	/* Re-initialize the MMU, e.g. to pick up CR4 MMU role changes. */
>   	kvm_init_mmu(vcpu);
> @@ -2600,16 +2620,6 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>   				from_vmentry, entry_failure_code))
>   		return -EINVAL;
>   
> -	/*
> -	 * Immediately write vmcs02.GUEST_CR3.  It will be propagated to vmcs12
> -	 * on nested VM-Exit, which can occur without actually running L2 and
> -	 * thus without hitting vmx_load_mmu_pgd(), e.g. if L1 is entering L2 with
> -	 * vmcs12.GUEST_ACTIVITYSTATE=HLT, in which case KVM will intercept the
> -	 * transition to HLT instead of running L2.
> -	 */
> -	if (enable_ept)
> -		vmcs_writel(GUEST_CR3, vmcs12->guest_cr3);
> -
>   	/* Late preparation of GUEST_PDPTRs now that EFER and CRs are set. */
>   	if (load_guest_pdptrs_vmcs12 && nested_cpu_has_ept(vmcs12) &&
>   	    is_pae_paging(vcpu)) {
> 

I have to think more about this one and patch 17.  I queued the rest.

Paolo