KVM: SVM: WARN if GATag generation drops VM or vCPU ID information

WARN if generating a GATag given a VM ID and vCPU ID doesn't yield the
same IDs when pulling the IDs back out of the tag.  Don't bother adding
error handling to callers, this is very much a paranoid sanity check as
KVM fully controls the VM ID and is supposed to reject too-big vCPU IDs.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Tested-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Message-Id: <20230207002156.521736-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/avic.c

@@ -43,12 +43,21 @@
 #define AVIC_VM_ID_SHIFT		HWEIGHT32(AVIC_PHYSICAL_MAX_INDEX_MASK)
 #define AVIC_VM_ID_MASK			(GENMASK(31, AVIC_VM_ID_SHIFT) >> AVIC_VM_ID_SHIFT)
 
-#define AVIC_GATAG(x, y)		(((x & AVIC_VM_ID_MASK) << AVIC_VM_ID_SHIFT) | \
-						(y & AVIC_VCPU_ID_MASK))
 #define AVIC_GATAG_TO_VMID(x)		((x >> AVIC_VM_ID_SHIFT) & AVIC_VM_ID_MASK)
 #define AVIC_GATAG_TO_VCPUID(x)		(x & AVIC_VCPU_ID_MASK)
 
-static_assert(AVIC_GATAG(AVIC_VM_ID_MASK, AVIC_VCPU_ID_MASK) == -1u);
+#define __AVIC_GATAG(vm_id, vcpu_id)	((((vm_id) & AVIC_VM_ID_MASK) << AVIC_VM_ID_SHIFT) | \
+					 ((vcpu_id) & AVIC_VCPU_ID_MASK))
+#define AVIC_GATAG(vm_id, vcpu_id)					\
+({									\
+	u32 ga_tag = __AVIC_GATAG(vm_id, vcpu_id);			\
+									\
+	WARN_ON_ONCE(AVIC_GATAG_TO_VCPUID(ga_tag) != (vcpu_id));	\
+	WARN_ON_ONCE(AVIC_GATAG_TO_VMID(ga_tag) != (vm_id));		\
+	ga_tag;								\
+})
+
+static_assert(__AVIC_GATAG(AVIC_VM_ID_MASK, AVIC_VCPU_ID_MASK) == -1u);
 
 static bool force_avic;
 module_param_unsafe(force_avic, bool, 0444);