Re: [PATCH 3/5] KVM: nVMX: Update VMX controls MSR according to guest CPUID after setting VMX MSRs

[Jim Mattson]

On Wed, Sep 2, 2020 at 11:16 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Fri, Aug 28, 2020 at 01:39:39PM -0700, Jim Mattson wrote:
> > On Fri, Aug 28, 2020 at 1:54 AM Chenyi Qiang <chenyi.qiang@xxxxxxxxx> wrote:
> > >
> > > Update the fields (i.e. VM_{ENTRY_LOAD, EXIT_CLEAR}_BNDCFGS and
> > > VM_{ENTRY, EXIT}_LOAD_IA32_PERF_GLOBAL_CTRL) in
> > > nested MSR_IA32_VMX_TRUE_{ENTRY, EXIT}_CTLS according to guest CPUID
> > > when user space initializes the features MSRs. Regardless of the order
> > > of SET_CPUID and SET_MSRS from the user space, do the update to avoid
> > > MSR values overriding.
> > >
> > > Signed-off-by: Chenyi Qiang <chenyi.qiang@xxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/vmx/vmx.c | 6 +++++-
> > >  1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 819c185adf09..f9664ccc003b 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -345,6 +345,7 @@ static bool guest_state_valid(struct kvm_vcpu *vcpu);
> > >  static u32 vmx_segment_access_rights(struct kvm_segment *var);
> > >  static __always_inline void vmx_disable_intercept_for_msr(unsigned long *msr_bitmap,
> > >                                                           u32 msr, int type);
> > > +static void nested_vmx_entry_exit_ctls_update(struct kvm_vcpu *vcpu);
> > >
> > >  void vmx_vmexit(void);
> > >
> > > @@ -2161,7 +2162,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> > >                         return 1; /* they are read-only */
> > >                 if (!nested_vmx_allowed(vcpu))
> > >                         return 1;
> > > -               return vmx_set_vmx_msr(vcpu, msr_index, data);
> > > +               ret = vmx_set_vmx_msr(vcpu, msr_index, data);
> > > +               nested_vmx_pmu_entry_exit_ctls_update(vcpu);
> > > +               nested_vmx_entry_exit_ctls_update(vcpu);
> > > +               break;
> >
> > Now I see what you're doing. This commit should probably come before
> > the previous commit, so that at no point in the series can userspace
> > set VMX MSR bits that should be cleared based on the guest CPUID.
> >
> > There's an ABI change here: userspace may no longer get -EINVAL if it
> > tries to set an illegal VMX MSR bit. Instead, some illegal bits are
> > silently cleared. Moreover, these functions will potentially set VMX
> > MSR bits that userspace has just asked to clear.
>
> Can we simply remove nested_vmx_entry_exit_ctls_update() and
> nested_vmx_pmu_entry_exit_ctls_update()?  It's userspace's responsibility
> to present a valid vCPU model to the guest, I don't see any reason to
> silently tweak the VMX MSRs unless allowing the bogus config breaks KVM.
> E.g. there are many more controls that are non-sensical without "native"
> support for the associated feature.

We might need a test for kvm_mpx_supported() here:

/* If not VM_EXIT_CLEAR_BNDCFGS, the L2 value propagates to L1.  */
if (vmcs12->vm_exit_controls & VM_EXIT_CLEAR_BNDCFGS)
        vmcs_write64(GUEST_BNDCFGS, 0);

BTW, where does the L2 value propagate to L1 if not VM_EXIT_CLEAR_BNDCFGS?