linux-kernel - Re: [PATCH v11 2/9] KVM: VMX: Set guest CET MSRs per KVM and host configuration

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Date:   Fri, 24 Apr 2020 07:55:29 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, jmattson@...gle.com,
        yu.c.zhang@...ux.intel.com
Subject: Re: [PATCH v11 2/9] KVM: VMX: Set guest CET MSRs per KVM and host
 configuration

On Fri, Apr 24, 2020 at 10:07:51PM +0800, Yang Weijiang wrote:
> On Thu, Apr 23, 2020 at 09:27:49AM -0700, Sean Christopherson wrote:
> > On Thu, Mar 26, 2020 at 04:18:39PM +0800, Yang Weijiang wrote:
> > > @@ -7102,6 +7138,10 @@ static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
> > >  			vmx_set_guest_msr(vmx, msr, enabled ? 0 : TSX_CTRL_RTM_DISABLE);
> > >  		}
> > >  	}
> > > +
> > > +	if (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
> > > +	    guest_cpuid_has(vcpu, X86_FEATURE_IBT))
> > > +		vmx_update_intercept_for_cet_msr(vcpu);
> > 
> > This is wrong, it will miss the case where userspace double configures CPUID
> > and goes from CET=1 to CET=0.  This should instead be:
> > 
> > 	if (supported_xss & (XFEATURE_MASK_CET_KERNEL | XFEATURE_MASK_CET_USER))
> > 		vmx_update_intercept_for_cet_msr(vcpu);
> > 
> > >  }
> Here CET=1/0, did you mean the CET bit in XSS or CR4.CET? If it's the
> former, then it's OK for me.

The former, i.e. update the CET MSRs if KVM supports CET virtualization and
the guest's CPUID configuration is changing.