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Subject: Re: [PATCH v2] KVM: nVMX: Consult only the "basic" exit reason when
 routing nested exit
To:     Sean Christopherson <sean.j.christopherson@intel.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, Xiaoyao Li <xiaoyao.li@intel.com>,
        Oliver Upton <oupton@google.com>,
        Krish Sadhukhan <krish.sadhukhan@oracle.com>,
        Miaohe Lin <linmiaohe@huawei.com>
References: <20200608191857.30319-1-sean.j.christopherson@intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <6153e63a-b68e-3537-8fab-d92163c329f0@redhat.com>
Date:   Tue, 9 Jun 2020 12:18:28 +0200
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On 08/06/20 21:18, Sean Christopherson wrote:
> Consult only the basic exit reason, i.e. bits 15:0 of vmcs.EXIT_REASON,
> when determining whether a nested VM-Exit should be reflected into L1 or
> handled by KVM in L0.
> 
> For better or worse, the switch statements nested_vmx_l0_wants_exit()
> and nested_vmx_l1_wants_exit() default to reflecting the VM-Exit into L1
> for any nested VM-Exit without dedicated logic.  Because the case
> statements only contain the basic exit reason, any VM-Exit with modifier
> bits set will be reflected to L1, even if KVM intended to handle it in
> L0.
> 
> Practically speaking, this only affects EXIT_REASON_MCE_DURING_VMENTRY,
> i.e. a #MC that occurs on nested VM-Enter would be incorrectly routed to
> L1, as "failed VM-Entry" is the only modifier that KVM can currently
> encounter.  The SMM modifiers will never be generated as KVM doesn't
> support/employ a SMI Transfer Monitor.  Ditto for "exit from enclave",
> as KVM doesn't yet support virtualizing SGX, i.e. it's impossible to
> enter an enclave in a KVM guest (L1 or L2).
> 
> Note, the original version of this fix[*] is functionally equivalent and
> far more suited to backporting as the affected code was refactored since
> the original patch was posted.
> 
> [*] https://lkml.kernel.org/r/20200227174430.26371-1-sean.j.christopherson@intel.com
> 
> Fixes: 644d711aa0e1 ("KVM: nVMX: Deciding if L0 or L1 should handle an L2 exit")
> Cc: Jim Mattson <jmattson@google.com>
> Cc: Xiaoyao Li <xiaoyao.li@intel.com>
> Cc: stable@vger.kernel.org
> Cc: Oliver Upton <oupton@google.com>
> Cc: Krish Sadhukhan <krish.sadhukhan@oracle.com>
> Cc: Miaohe Lin <linmiaohe@huawei.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> 
> Another wounded soldier. 
> 
> Oliver, Krish, and Miaohe all provided reviews for v1, but I didn't feel
> comfortable adding the tags to v2 because this is far from a straight
> rebase.
> 
> v2: Rebased to kvm/queue, commit fb7333dfd812 ("KVM: SVM: fix calls ...").
> 
>  arch/x86/kvm/vmx/nested.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index bcb50724be38..adb11b504d5c 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -5672,7 +5672,7 @@ static bool nested_vmx_l0_wants_exit(struct kvm_vcpu *vcpu, u32 exit_reason)
>  {
>  	u32 intr_info;
>  
> -	switch (exit_reason) {
> +	switch ((u16)exit_reason) {
>  	case EXIT_REASON_EXCEPTION_NMI:
>  		intr_info = vmx_get_intr_info(vcpu);
>  		if (is_nmi(intr_info))
> @@ -5733,7 +5733,7 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu, u32 exit_reason)
>  	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
>  	u32 intr_info;
>  
> -	switch (exit_reason) {
> +	switch ((u16)exit_reason) {
>  	case EXIT_REASON_EXCEPTION_NMI:
>  		intr_info = vmx_get_intr_info(vcpu);
>  		if (is_nmi(intr_info))
> 

Thanks, queued with git magic (committed v1 based on v5.7, merged into
kvm/queue, added the delta to v2 in the merge commit) so that stable
branches can cherry-pick cleanly.

Paolo