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Subject: Re: [PATCH v5 2/2] KVM: X86: Add Force Emulation Prefix for "emulate
 the next instruction"
To:     Wanpeng Li <kernellwp@gmail.com>,
        David Hildenbrand <david@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        =?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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From:   Paolo Bonzini <pbonzini@redhat.com>
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Message-ID: <f8da5618-6d80-e209-fb28-0dc11cee5c1d@redhat.com>
Date:   Wed, 4 Apr 2018 19:09:19 +0200
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On 04/04/2018 15:35, Wanpeng Li wrote:
> 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.

But David's observation is still better because your code doesn't handle
usermode exits.  I've fixed this up.

Paolo