linux-kernel - Re: [PATCH v2] KVM: nVMX: Consult only the "basic" exit reason when routing nested exit

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Message-ID: <CAOQ_QsjEnvk+ci8AAjytoFd+x8x3Li+iJjfT0gd=nMP036G9VQ@mail.gmail.com>
Date:   Mon, 8 Jun 2020 12:38:03 -0700
From:   Oliver Upton <oupton@...gle.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 list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Krish Sadhukhan <krish.sadhukhan@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>
Subject: Re: [PATCH v2] KVM: nVMX: Consult only the "basic" exit reason when
 routing nested exit

On Mon, Jun 8, 2020 at 12:19 PM Sean Christopherson
<sean.j.christopherson@...el.com> 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@...gle.com>
> Cc: Xiaoyao Li <xiaoyao.li@...el.com>
> Cc: stable@...r.kernel.org
> Cc: Oliver Upton <oupton@...gle.com>
> Cc: Krish Sadhukhan <krish.sadhukhan@...cle.com>
> Cc: Miaohe Lin <linmiaohe@...wei.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Reviewed-by: Oliver Upton <oupton@...gle.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))
> --
> 2.26.0
>