linux-kernel - Re: [PATCH 5/5] KVM: nSVM: set a dummy exit reason in L1 vmcb when loading the nested state

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Message-ID: <a3a08464-d67c-fe71-6b2a-01b9ee58312c@redhat.com>
Date:   Mon, 3 May 2021 16:02:42 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Cathy Avery <cavery@...hat.com>
Subject: Re: [PATCH 5/5] KVM: nSVM: set a dummy exit reason in L1 vmcb when
 loading the nested state

On 03/05/21 14:54, Maxim Levitsky wrote:
> Since the nested migration is a result of a VMRUN, this makes it
> possible to keep a warning that L1 vmcb should always have
> VMRUN exit reason when switching back to it, which
> otherwise triggers incorrectly.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>

Does this fix "KVM: nSVM: If VMRUN is single-stepped, queue the #DB 
intercept in nested_svm_vmexit()"?  I don't like this, and also vmcb12 
is not initialized here (nested_load_control_from_vmcb12 is using the 
state passed in from userspace instead).

I think you should just remove the WARN instead.

Paolo

> ---
>   arch/x86/kvm/svm/nested.c | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 12a12ae940fa..146be4b5084b 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1338,6 +1338,12 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
>   	svm->vmcb01.ptr->save.rip = save->rip;
>   	svm->vmcb01.ptr->save.cpl = 0;
>   
> +	/*
> +	 * For consistency sake, restore the L1 exit reason
> +	 * (that happened prior to the migration) to SVM_EXIT_VMRUN.
> +	 */
> +	svm->vmcb->control.exit_code = SVM_EXIT_VMRUN;
> +
>   	nested_load_control_from_vmcb12(svm, ctl);
>   
>   	svm_switch_vmcb(svm, &svm->nested.vmcb02);
>