linux-kernel - Re: [PATCH v2 4/7] KVM: nVMX: Force enlightened VMCS sync from nested_vmx

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Date:   Mon, 24 May 2021 15:27:47 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/7] KVM: nVMX: Force enlightened VMCS sync from
 nested_vmx_failValid()

On Mon, 2021-05-17 at 15:50 +0200, Vitaly Kuznetsov wrote:
> 'need_vmcs12_to_shadow_sync' is used for both shadow and enlightened
> VMCS sync when we exit to L1. The comment in nested_vmx_failValid()
> validly states why shadow vmcs sync can be omitted but this doesn't
> apply to enlightened VMCS as it 'shadows' all VMCS12 fields.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index ec476f64df73..eb2d25a93356 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -194,9 +194,13 @@ static int nested_vmx_failValid(struct kvm_vcpu *vcpu,
>  			| X86_EFLAGS_ZF);
>  	get_vmcs12(vcpu)->vm_instruction_error = vm_instruction_error;
>  	/*
> -	 * We don't need to force a shadow sync because
> -	 * VM_INSTRUCTION_ERROR is not shadowed
> +	 * We don't need to force sync to shadow VMCS because
> +	 * VM_INSTRUCTION_ERROR is not shadowed. Enlightened VMCS 'shadows' all
> +	 * fields and thus must be synced.
>  	 */
> +	if (nested_evmcs_is_used(to_vmx(vcpu)))
> +		to_vmx(vcpu)->nested.need_vmcs12_to_shadow_sync = true;

Cool, this is a bug that I noticed too while reviewing
previous patches in this series.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky

> +
>  	return kvm_skip_emulated_instruction(vcpu);
>  }
>