Re: [PATCH 2/3] KVM: x86: emulator: Return to user-mode on L1 CPL=0 emulation failure

[Liran Alon <LIRAN.ALON@ORACLE.COM>]

On 06/11/17 15:50, Paolo Bonzini wrote:
> On 06/11/2017 14:25, Liran Alon wrote:
>>> What is the caller of x86_emulate_instruction that you are interested
>>> in?  #UD is not affected, because emulation_type has EMULTYPE_TRAP_UD
>>> set and therefore x86_emulate_instruction exits before invoking
>>> handle_emulation_failure.
>>>
>> I think this patch is still correct from a number of reasons:
>>
>> 1) If I understand the code correctly, semantically it doesn't make
>> sense to fill kvm_run struct without exiting to user-mode. Therefore, if
>> emulator filled kvm_run, it makes sense that it needs to return
>> EMULATE_USER_EXIT.
>>
>> 2) EMULTYPE_TRAP_UD only causes the emulator to return EMULATE_FAIL in
>> case emulation fails on instruction decoding. However, consider a case
>> where #UD intercept happens on a valid instruction (such as VMMCALL AMD
>> opcode on physical Intel CPU). In that case, instruction decoding
>> doesn't fail but we could still fail on instruction emulation at
>> x86_emulate_insn(). In this case, EMULTYPE_TRAP_UD flag is not
>> considered anymore and failure will reach handle_emulation_failure().
>
> (1) is true, but (2) more or less answers my question.  So
>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
>
>> 3) We have another KVM commits series (not upstream yet) that adds
>> intercept on #GP which calls the x86 emulator. This is done to allow
>> access to I/O ports even though they aren't allowed via guest's TSS I/O
>> permissions bitmap.
>
> vmport?... :)
Yep. Stay tuned. :)
>
> Paolo
>