Re: [PATCH v2] KVM: nVMX/nSVM: Don't intercept #UD when running L2

Paolo Bonzini
2017-11-06T14:16:38Z

On 06/11/2017 15:15, Liran Alon wrote:
> When running L2, #UD should be intercepted by L1 or just forwarded
> directly to L2. It should not reach L0 x86 emulator.
> Therefore, set intercept for #UD only based on L1 exception-bitmap.
>
> Also add WARN_ON_ONCE() on L0 #UD intercept handlers to make sure
> it is never reached while running L2.
>
> This improves commit ae1f57670703 ("KVM: nVMX: Do not emulate #UD while
> in guest mode") by removing an unnecessary exit from L2 to L0 on #UD
> when L1 doesn't intercept it.
>
> In addition, SVM L0 #UD intercept handler doesn't handle correctly the
> case it is raised from L2. In this case, it should forward the #UD to
> guest instead of x86 emulator. As done in VMX #UD intercept handler.
> This commit fixes this issue as-well.
>
> Signed-off-by: Liran Alon <liran.alon@oracle.com>
> Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> ---
> arch/x86/kvm/svm.c | 9 ++++++++-
> arch/x86/kvm/vmx.c | 9 ++++-----
> 2 files changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 0e68f0b3cbf7..e49d44afbe9a 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -361,6 +361,7 @@ static void recalc_intercepts(struct vcpu_svm *svm)
> {
> struct vmcb_control_area *c, *h;
> struct nested_state *g;
> + u32 h_intercept_exceptions;
>
> mark_dirty(svm->vmcb, VMCB_INTERCEPTS);
>
> @@ -371,9 +372,14 @@ static void recalc_intercepts(struct vcpu_svm *svm)
> h = &svm->nested.hsave->control;
> g = &svm->nested;
>
> + /* No need to intercept #UD if L1 doesn't intercept it */
> + h_intercept_exceptions =
> + h->intercept_exceptions & ~(1U << UD_VECTOR);
> +
> c->intercept_cr = h->intercept_cr | g->intercept_cr;
> c->intercept_dr = h->intercept_dr | g->intercept_dr;
> - c->intercept_exceptions = h->intercept_exceptions | g->intercept_exceptions;
> + c->intercept_exceptions =
> + h_intercept_exceptions | g->intercept_exceptions;
> c->intercept = h->intercept | g->intercept;
> }
>
> @@ -2188,6 +2194,7 @@ static int ud_interception(struct vcpu_svm *svm)
> {
> int er;
>
> + WARN_ON_ONCE(is_guest_mode(&svm->vcpu));
> er = emulate_instruction(&svm->vcpu, EMULTYPE_TRAP_UD);
> if (er != EMULATE_DONE)
> kvm_queue_exception(&svm->vcpu, UD_VECTOR);
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 95a01609d7ee..dfa856d31b17 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -1878,7 +1878,7 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
> {
> u32 eb;
>
> - eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR) |
> + eb = (1u << PF_VECTOR) | (1u << MC_VECTOR) |
> (1u << DB_VECTOR) | (1u << AC_VECTOR);
> if ((vcpu->guest_debug &
> (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP)) ==
> @@ -1896,6 +1896,8 @@ static void update_exception_bitmap(struct kvm_vcpu *vcpu)
> */
> if (is_guest_mode(vcpu))
> eb |= get_vmcs12(vcpu)->exception_bitmap;
> + else
> + eb |= 1u << UD_VECTOR;
>
> vmcs_write32(EXCEPTION_BITMAP, eb);
> }
> @@ -5881,10 +5883,7 @@ static int handle_exception(struct kvm_vcpu *vcpu)
> return 1; /* already handled by vmx_vcpu_run() */
>
> if (is_invalid_opcode(intr_info)) {
> - if (is_guest_mode(vcpu)) {
> - kvm_queue_exception(vcpu, UD_VECTOR);
> - return 1;
> - }
> + WARN_ON_ONCE(is_guest_mode(vcpu));
> er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
> if (er != EMULATE_DONE)
> kvm_queue_exception(vcpu, UD_VECTOR);
>

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>