KVM: x86: Drop special XSAVE handling from guest_cpuid_has()

Now that KVM prevents setting host-reserved CR4 bits, drop the dedicated
XSAVE check in guest_cpuid_has() in favor of open coding similar checks
in the SVM/VMX XSAVES enabling flows.

Note, checking boot_cpu_has(X86_FEATURE_XSAVE) in the XSAVES flows is
technically redundant with respect to the CR4 reserved bit checks, e.g.
XSAVES #UDs if CR4.OSXSAVE=0 and arch.xsaves_enabled is consumed if and
only if CR4.OXSAVE=1 in guest.  Keep (add?) the explicit boot_cpu_has()
checks to help document KVM's usage of arch.xsaves_enabled.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.h

@@ -93,10 +93,6 @@ static __always_inline bool guest_cpuid_has(struct kvm_vcpu *vcpu, unsigned x86_
 {
 	int *reg;
 
-	if (x86_feature == X86_FEATURE_XSAVE &&
-			!static_cpu_has(X86_FEATURE_XSAVE))
-		return false;
-
 	reg = guest_cpuid_get_register(vcpu, x86_feature);
 	if (!reg)
 		return false;

arch/x86/kvm/svm.c

@@ -5917,6 +5917,7 @@ static void svm_cpuid_update(struct kvm_vcpu *vcpu)
 	struct vcpu_svm *svm = to_svm(vcpu);
 
 	vcpu->arch.xsaves_enabled = guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
+				    boot_cpu_has(X86_FEATURE_XSAVE) &&
 				    boot_cpu_has(X86_FEATURE_XSAVES);
 
 	/* Update nrips enabled cache */

arch/x86/kvm/vmx/vmx.c

@@ -4049,6 +4049,7 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
 	if (vmx_xsaves_supported()) {
 		/* Exposing XSAVES only when XSAVE is exposed */
 		bool xsaves_enabled =
+			boot_cpu_has(X86_FEATURE_XSAVE) &&
 			guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
 			guest_cpuid_has(vcpu, X86_FEATURE_XSAVES);