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

[Sean Christopherson <sean.j.christopherson@intel.com>]

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@intel.com> 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).