linux-kernel - Re: [PATCH] KVM: nVMX: Consult only the "basic" exit reason when routing nested exit

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Date:   Fri, 28 Feb 2020 01:50:44 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH] KVM: nVMX: Consult only the "basic" exit reason when
 routing nested exit

Sean Christopherson <sean.j.christopherson@...el.com> writes:
>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 statement in nested_vmx_exit_reflected() currently defaults to "true", i.e. reflects 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).
>

Nice catch! There are similar patch catching this exit reson error.
Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>

>Fixes: 644d711aa0e1 ("KVM: nVMX: Deciding if L0 or L1 should handle an L2 exit")
>Cc: Jim Mattson <jmattson@...gle.com>
>Cc: Xiaoyao Li <xiaoyao.li@...el.com>
>Cc: stable@...r.kernel.org
>Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
>---