Re: [PATCH v2 4/5] KVM: nVMX: Emulate MTF when performing instruction emulation

Sean Christopherson
2021-08-13T16:35:43Z

On Thu, Aug 12, 2021, Jim Mattson wrote:
> On Tue, Jan 28, 2020 at 1:27 AM Oliver Upton <oupton@google.com> wrote:
> >
> > Since commit 5f3d45e7f282 ("kvm/x86: add support for
> > MONITOR_TRAP_FLAG"), KVM has allowed an L1 guest to use the monitor trap
> > flag processor-based execution control for its L2 guest. KVM simply
> > forwards any MTF VM-exits to the L1 guest, which works for normal
> > instruction execution.
> >
> > However, when KVM needs to emulate an instruction on the behalf of an L2
> > guest, the monitor trap flag is not emulated. Add the necessary logic to
> > kvm_skip_emulated_instruction() to synthesize an MTF VM-exit to L1 upon
> > instruction emulation for L2.
> >
> > Fixes: 5f3d45e7f282 ("kvm/x86: add support for MONITOR_TRAP_FLAG")
> > Signed-off-by: Oliver Upton <oupton@google.com>
> > ---

...

> > diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> > index 503d3f42da16..3f3f780c8c65 100644
> > --- a/arch/x86/include/uapi/asm/kvm.h
> > +++ b/arch/x86/include/uapi/asm/kvm.h
> > @@ -390,6 +390,7 @@ struct kvm_sync_regs {
> > #define KVM_STATE_NESTED_GUEST_MODE 0x00000001
> > #define KVM_STATE_NESTED_RUN_PENDING 0x00000002
> > #define KVM_STATE_NESTED_EVMCS 0x00000004
> > +#define KVM_STATE_NESTED_MTF_PENDING 0x00000008
>
> Maybe I don't understand the distinction, but shouldn't this new flag
> have a KVM_STATE_NESTED_VMX prefix and live with
> KVM_STATE_VMX_PREEMPTION_TIMER_DEADLINE, below?

That does seem to be the case, seems highly unlikely SVM will add MTF. And SVM's
KVM_STATE_NESTED_GIF_SET should have been SVM specific, but kvm_svm_nested_state_hdr
doesn't even reserve a flags field :-/

> > #define KVM_STATE_NESTED_SMM_GUEST_MODE 0x00000001
> > #define KVM_STATE_NESTED_SMM_VMXON 0x00000002