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SubjectRe: [PATCH v5 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"
2018-04-05 1:09 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
> 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.

My code handles it, return emulate_instruction(vcpu, 0) ==
EMULATE_DONE, it will return 0 since EMULATE_USER_EXIT == EMULATE_DONE
fails.

> I've fixed this up.

Thanks. The codes similar to my v3 but more beauty. :) I change to
this view since Radim's comments to v3
https://www.spinics.net/lists/kvm/msg166999.html

Regards,
Wanpeng Li

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