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    SubjectRe: [PATCH v5 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"
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    > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    > index 1eb495e..a55ecef 100644
    > --- a/arch/x86/kvm/x86.c
    > +++ b/arch/x86/kvm/x86.c
    > @@ -146,6 +146,9 @@ bool __read_mostly enable_vmware_backdoor = false;
    > module_param(enable_vmware_backdoor, bool, S_IRUGO);
    > EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
    >
    > +static bool __read_mostly force_emulation_prefix = false;
    > +module_param(force_emulation_prefix, bool, S_IRUGO);
    > +
    > #define KVM_NR_SHARED_MSRS 16
    >
    > struct kvm_shared_msrs_global {
    > @@ -4844,6 +4847,21 @@ int handle_ud(struct kvm_vcpu *vcpu)
    > {
    > enum emulation_result er;
    >
    > + if (force_emulation_prefix) {
    > + char sig[5]; /* ud2; .ascii "kvm" */
    > + struct x86_exception e;
    > +
    > + if (kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
    > + kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e))
    > + goto emulate_ud;
    > +
    > + if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
    > + kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
    > + return emulate_instruction(vcpu, 0) == EMULATE_DONE;

    What if we would have an invalid instruction here? Shouldn't you handle
    the emulate_instruction() like below?
    (e.g. keep a variable with the emulation type (0 vs EMULTYPE_TRAP_UD)
    and reuse emulate_ud below)

    > + }
    > + }
    > +
    > +emulate_ud:
    > er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
    > if (er == EMULATE_USER_EXIT)
    > return 0;
    >


    --

    Thanks,

    David / dhildenb

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