Re: [PATCH] KVM: VMX: expose the host's ARCH_CAPABILITIES MSR to userspace

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Mon, Feb 26, 2018 at 05:13:00PM -0500, Konrad Rzeszutek Wilk wrote:
> On Sat, Feb 24, 2018 at 01:52:26AM +0100, Paolo Bonzini wrote:
> > Use the new MSR feature framework to expose the ARCH_CAPABILITIES MSR to
> > userspace.  This way, userspace can access the capabilities even if it
> > does not have the permissions to read MSRs.
> 
> ... That is good but could you expand a bit of why it would want this?
> 
> I am 99% sure it is due to the lovely spectre_v2 mitigation but
> could you include that in the commit message so that in say a year
> folks would know what this is?
> 
> Also what branch is this based on? I am not seeing this vmx_get_msr_feature
> in kvm/master or kvm/linux-next ?

Ah I see you posted them! Sorry for that particular noise.
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >  arch/x86/kvm/vmx.c | 12 +++++++++++-
> >  arch/x86/kvm/x86.c |  1 +
> >  2 files changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index 130fca0ea1bf..99689061e11e 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -3228,7 +3228,17 @@ static inline bool vmx_feature_control_msr_valid(struct kvm_vcpu *vcpu,
> >  
> >  static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
> >  {
> > -	return 1;
> > +	switch (msr->index) {
> > +	case MSR_IA32_ARCH_CAPABILITIES:
> > +		if (!boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
> > +			return 1;
> > +		rdmsrl(msr->index, msr->data);
> > +		break;
> > +	default:
> > +		return 1;
> > +	}
> > +
> > +	return 0;
> >  }
> >  
> >  /*
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 54b4ed55945b..e9a8cc9e3b2b 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1054,6 +1054,7 @@ bool kvm_rdpmc(struct kvm_vcpu *vcpu)
> >   * can be used by a hypervisor to validate requested CPU features.
> >   */
> >  static u32 msr_based_features[] = {
> > +	MSR_IA32_ARCH_CAPABILITIES,
> >  	MSR_F10H_DECFG,
> >  };
> >  
> > -- 
> > 1.8.3.1
> >