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    SubjectRe: [PATCH v6 5/8] KVM: VMX: Load Guest CET via VMCS when CET is enabled in Guest
    On Thu, Jul 25, 2019 at 11:12:43AM +0800, Yang Weijiang wrote:
    > "Load Guest CET state" bit controls whether Guest CET states
    > will be loaded at Guest entry. Before doing that, KVM needs
    > to check if CPU CET feature is enabled on host and available
    > to Guest.
    >
    > Note: SHSTK and IBT features share one control MSR:
    > MSR_IA32_{U,S}_CET, which means it's difficult to hide
    > one feature from another in the case of SHSTK != IBT,
    > after discussed in community, it's agreed to allow Guest
    > control two features independently as it won't introduce
    > security hole.
    >
    > Co-developed-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
    > Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
    > Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
    > ---
    > arch/x86/kvm/vmx/vmx.c | 13 +++++++++++++
    > 1 file changed, 13 insertions(+)
    >
    > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
    > index ce5d1e45b7a5..fbf9c335cf7b 100644
    > --- a/arch/x86/kvm/vmx/vmx.c
    > +++ b/arch/x86/kvm/vmx/vmx.c
    > @@ -44,6 +44,7 @@
    > #include <asm/spec-ctrl.h>
    > #include <asm/virtext.h>
    > #include <asm/vmx.h>
    > +#include <asm/cet.h>
    >
    > #include "capabilities.h"
    > #include "cpuid.h"
    > @@ -2923,6 +2924,18 @@ int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
    > if (!nested_vmx_allowed(vcpu) || is_smm(vcpu))
    > return 1;
    > }
    > + if (cpu_x86_cet_enabled() &&

    It'd probably be better to check a KVM function here, e.g. a wrapper of
    kvm_supported_xss(). I don't think it will ever matter, but it'd be nice
    to have a single kill switch given the variety of different enable bits
    for CET.

    > + (guest_cpuid_has(vcpu, X86_FEATURE_SHSTK) ||
    > + guest_cpuid_has(vcpu, X86_FEATURE_IBT))) {
    > + if (cr4 & X86_CR4_CET)
    > + vmcs_set_bits(VM_ENTRY_CONTROLS,
    > + VM_ENTRY_LOAD_GUEST_CET_STATE);
    > + else
    > + vmcs_clear_bits(VM_ENTRY_CONTROLS,
    > + VM_ENTRY_LOAD_GUEST_CET_STATE);
    > + } else if (cr4 & X86_CR4_CET) {
    > + return 1;
    > + }
    >
    > if (to_vmx(vcpu)->nested.vmxon && !nested_cr4_valid(vcpu, cr4))
    > return 1;
    > --
    > 2.17.2
    >

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