KVM: selftests: Add support for #DE exception fixup

Add support for handling #DE (divide error) exceptions in KVM selftests
so that the fastops test can verify KVM correctly handles #DE when
emulating DIV or IDIV on behalf of the guest.  Morph #DE to 0xff (i.e.
to -1) as a mostly-arbitrary vector to indicate #DE, so that '0' (the
real #DE vector) can still be used to indicate "no exception".

Link: https://lore.kernel.org/r/20250909202835.333554-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/include/x86/processor.h

@@ -1179,6 +1179,12 @@ struct idt_entry {
 void vm_install_exception_handler(struct kvm_vm *vm, int vector,
 			void (*handler)(struct ex_regs *));
 
+/*
+ * Exception fixup morphs #DE to an arbitrary magic vector so that '0' can be
+ * used to signal "no expcetion".
+ */
+#define KVM_MAGIC_DE_VECTOR 0xff
+
 /* If a toddler were to say "abracadabra". */
 #define KVM_EXCEPTION_MAGIC 0xabacadabaULL
 

tools/testing/selftests/kvm/lib/x86/processor.c

@@ -557,7 +557,7 @@ static bool kvm_fixup_exception(struct ex_regs *regs)
 		return false;
 
 	if (regs->vector == DE_VECTOR)
-		return false;
+		regs->vector = KVM_MAGIC_DE_VECTOR;
 
 	regs->rip = regs->r11;
 	regs->r9 = regs->vector;