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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Wed, 4 Apr 2018 21:35:57 +0800
Message-ID: <CANRm+CwMRb9hbV5PjGMoTe1oDa+cEZ0ex7N-1J5NkWiCV62M4A@mail.gmail.com>
Subject: Re: [PATCH v5 2/2] KVM: X86: Add Force Emulation Prefix for "emulate
 the next instruction"
To:     David Hildenbrand <david@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Liran Alon <liran.alon@oracle.com>
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2018-04-04 19:59 GMT+08:00 David Hildenbrand <david@redhat.com>:
>
>> 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_instruction(vcpu, 0) can handle invalid instruction.

Regards,
Wanpeng Li