RE: [PATCH v4 14/21] kvm: x86: Disable RDMSR interception of IA32_XFD_ERR

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Sean Christopherson <seanjc@google.com>
> Sent: Wednesday, January 5, 2022 3:35 AM
> 
> On Wed, Dec 29, 2021, Yang Zhong wrote:
> > From: Jing Liu <jing2.liu@intel.com>
> >
> > Disable read emulation of IA32_XFD_ERR MSR if guest cpuid includes XFD.
> > This saves one unnecessary VM-exit in guest #NM handler, given that the
> > MSR is already restored with the guest value before the guest is resumed.
> >
> > Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> > Signed-off-by: Jing Liu <jing2.liu@intel.com>
> > Signed-off-by: Yang Zhong <yang.zhong@intel.com>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 6 ++++++
> >  arch/x86/kvm/vmx/vmx.h | 2 +-
> >  2 files changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 4e51de876085..638665b3e241 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -162,6 +162,7 @@ static u32
> vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
> >  	MSR_FS_BASE,
> >  	MSR_GS_BASE,
> >  	MSR_KERNEL_GS_BASE,
> > +	MSR_IA32_XFD_ERR,
> >  #endif
> >  	MSR_IA32_SYSENTER_CS,
> >  	MSR_IA32_SYSENTER_ESP,
> > @@ -7228,6 +7229,11 @@ static void vmx_vcpu_after_set_cpuid(struct
> kvm_vcpu *vcpu)
> >  		}
> >  	}
> >
> > +	if (boot_cpu_has(X86_FEATURE_XFD))
> 
> This should be kvm_cpu_cap_has(), not boot_cpu_has().  If 32-bit kernels
> don't
> suppress XFD in boot_cpu_data, then using boot_cpus_has() is wrong.  And
> even if
> XFD is suppressed, using kvm_cpu_cap_has() is still preferable.
> 

Make sense.