Re: [RFC PATCH 4/4] KVM: vmx: add support for emulating UMIP

[Yang Zhang]

On 2016/7/13 17:35, Paolo Bonzini wrote:
> On 13/07/2016 11:21, Yang Zhang wrote:
> > > +    if ((cr4 & X86_CR4_UMIP) && !boot_cpu_has(X86_FEATURE_UMIP)) {
> > > +        vmcs_set_bits(SECONDARY_VM_EXEC_CONTROL,
> > > +                  SECONDARY_EXEC_DESC);
> > > +        hw_cr4 &= ~X86_CR4_UMIP;
> > > +    } else
> > > +        vmcs_clear_bits(SECONDARY_VM_EXEC_CONTROL,
> > > +                SECONDARY_EXEC_DESC);
> > > +
> >
> > Since the faults based on privilege level have priority over VM exits.
> > So we don't need to enable/disable SECONDARY_EXEC_DESC dynamically.
> > Instead, we can set it unconditionally.
>
> I'm setting it dynamically because it slows down LGDT, LLDT, LIDT and LTR.

You are right. And SGDT, SIDT, SLDT, SMSW, STR also will be intercepted 
even in CPL 0 if we don't disable it.

--
Yang
Alibaba Cloud Computing