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Subject: Re: [PATCH v2 08/21] KVM: nVMX: Ignore SIPI that arrives in L2 when
 vCPU is not in WFS
From:   Maxim Levitsky <mlevitsk@redhat.com>
To:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Oliver Upton <oupton@google.com>,
        Peter Shier <pshier@google.com>
Date:   Wed, 06 Jul 2022 14:59:24 +0300
In-Reply-To: <20220614204730.3359543-9-seanjc@google.com>
References: <20220614204730.3359543-1-seanjc@google.com>
         <20220614204730.3359543-9-seanjc@google.com>
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On Tue, 2022-06-14 at 20:47 +0000, Sean Christopherson wrote:
> Fall through to handling other pending exception/events for L2 if SIPI
> is pending while the CPU is not in Wait-for-SIPI.  KVM correctly ignores
> the event, but incorrectly returns immediately, e.g. a SIPI coincident
> with another event could lead to KVM incorrectly routing the event to L1
> instead of L2.
> 
> Fixes: bf0cd88ce363 ("KVM: x86: emulate wait-for-SIPI and SIPI-VMExit")
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index e794791a6bdd..d080bfca16ef 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3936,10 +3936,12 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
>  			return -EBUSY;
>  
>  		clear_bit(KVM_APIC_SIPI, &apic->pending_events);
> -		if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED)
> +		if (vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
>  			nested_vmx_vmexit(vcpu, EXIT_REASON_SIPI_SIGNAL, 0,
>  						apic->sipi_vector & 0xFFUL);
> -		return 0;
> +			return 0;
> +		}
> +		/* Fallthrough, the SIPI is completely ignored. */
>  	}
>  
>  	/*


Makes sense.

Note that svm_check_nested_events lacks the code to check for SIPI at all,
but SVM lacks SIPI intercept, thus this is likely correct, 
the place which delivers SIPI to L1 is I think kvm_apic_accept_events,
and it will ignore it unless the CPU is in INIT state, in which
it will not be in nested mode.


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

Best regards,
	Maxim Levitsky