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

[Jim Mattson]

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.

>         case MSR_IA32_RTIT_CTL:
>                 if (!vmx_pt_mode_is_host_guest() ||
>                         vmx_rtit_ctl_check(vcpu, data) ||
> --
> 2.17.1
>