KVM: x86: fix KVM_SET_XCRS for CPUs that do not support XSAVE

The KVM_SET_XCRS ioctl must accept anything that KVM_GET_XCRS
could return.  XCR0's bit 0 is always 1 in real processors with
XSAVE, and KVM_GET_XCRS will always leave bit 0 set even if the
emulated processor does not have XSAVE.  So, KVM_SET_XCRS must
ignore that bit when checking for attempts to enable unsupported
save states.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -577,6 +577,7 @@ static void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
 int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
 {
 	u64 xcr0;
+	u64 valid_bits;
 
 	/* Only support XCR_XFEATURE_ENABLED_MASK(xcr0) now  */
 	if (index != XCR_XFEATURE_ENABLED_MASK)
@@ -586,8 +587,16 @@ int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
 		return 1;
 	if ((xcr0 & XSTATE_YMM) && !(xcr0 & XSTATE_SSE))
 		return 1;
-	if (xcr0 & ~vcpu->arch.guest_supported_xcr0)
+
+	/*
+	 * Do not allow the guest to set bits that we do not support
+	 * saving.  However, xcr0 bit 0 is always set, even if the
+	 * emulated CPU does not support XSAVE (see fx_init).
+	 */
+	valid_bits = vcpu->arch.guest_supported_xcr0 | XSTATE_FP;
+	if (xcr0 & ~valid_bits)
 		return 1;
+
 	kvm_put_guest_xcr0(vcpu);
 	vcpu->arch.xcr0 = xcr0;
 	return 0;