Re: [RFC PATCH 08/21] KVM: x86: Add kvm_x86_ops hook to short circuit emulation

[Andy Lutomirski]

>> On Jul 29, 2019, at 7:49 PM, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>> 
>> On Sat, Jul 27, 2019 at 10:38:03AM -0700, Andy Lutomirski wrote:
>> On Fri, Jul 26, 2019 at 10:52 PM Sean Christopherson
>> <sean.j.christopherson@xxxxxxxxx> wrote:
>>> 
>>> Similar to the existing AMD #NPF case where emulation of the current
>>> instruction is not possible due to lack of information, virtualization
>>> of Intel SGX will introduce a scenario where emulation is not possible
>>> due to the VMExit occurring in an SGX enclave.  And again similar to
>>> the AMD case, emulation can be initiated by kvm_mmu_page_fault(), i.e.
>>> outside of the control of the vendor-specific code.
>>> 
>>> While the cause and architecturally visible behavior of the two cases
>>> is different,  e.g. Intel SGX will inject a #UD whereas AMD #NPF is a
>>> clean resume or complete shutdown, the impact on the common emulation
>>> code is identical: KVM must stop emulation immediately and resume the
>>> guest.
>>> 
>>> Replace the exisiting need_emulation_on_page_fault() with a more generic
>>> is_emulatable() kvm_x86_ops callback, which is called unconditionally
>>> by x86_emulate_instruction().
>> 
>> Having recently noticed that emulate_ud() is broken when the guest's
>> TF is set, I suppose I should ask: does your new code function
>> sensibly when TF is set?
> 
> Barring a VMX fault injection interaction I'm not thinking of, yes.  The
> SGX reaction to the #UD VM-Exit is to inject a #UD and resume the guest,
> pending breakpoints shouldn't be affected in any way (unless some other
> part of KVM mucks with them, e.g. when guest single-stepping is enabled).

What I mean is: does the code actually do what you think it does if TF is set?  Right now, as I understand it, the KVM emulation code has a bug in which some emulated faults also inject #DB despite the fact that the instruction faulted, and the #DB seems to take precedence over the original fault.  This confuses the guest.