kvm/emulate: fix a -Werror=cast-function-type

arch/x86/kvm/emulate.c: In function 'x86_emulate_insn':
arch/x86/kvm/emulate.c:5686:22: error: cast between incompatible
function types from 'int (*)(struct x86_emulate_ctxt *)' to 'void
(*)(struct fastop *)' [-Werror=cast-function-type]
    rc = fastop(ctxt, (fastop_t)ctxt->execute);

Fix it by using an unnamed union of a (*execute) function pointer and a
(*fastop) function pointer.

Fixes: 3009afc6 ("KVM: x86: Use a typedef for fastop functions")
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_emulate.h

@@ -292,6 +292,14 @@ enum x86emul_mode {
 #define X86EMUL_SMM_MASK             (1 << 6)
 #define X86EMUL_SMM_INSIDE_NMI_MASK  (1 << 7)
 
+/*
+ * fastop functions are declared as taking a never-defined fastop parameter,
+ * so they can't be called from C directly.
+ */
+struct fastop;
+
+typedef void (*fastop_t)(struct fastop *);
+
 struct x86_emulate_ctxt {
 	const struct x86_emulate_ops *ops;
 
@@ -324,7 +332,10 @@ struct x86_emulate_ctxt {
 	struct operand src;
 	struct operand src2;
 	struct operand dst;
+	union {
 		int (*execute)(struct x86_emulate_ctxt *ctxt);
+		fastop_t fop;
+	};
 	int (*check_perm)(struct x86_emulate_ctxt *ctxt);
 	/*
 	 * The following six fields are cleared together,

arch/x86/kvm/emulate.c

@@ -191,25 +191,6 @@
 #define NR_FASTOP (ilog2(sizeof(ulong)) + 1)
 #define FASTOP_SIZE 8
 
-/*
- * fastop functions have a special calling convention:
- *
- * dst:    rax        (in/out)
- * src:    rdx        (in/out)
- * src2:   rcx        (in)
- * flags:  rflags     (in/out)
- * ex:     rsi        (in:fastop pointer, out:zero if exception)
- *
- * Moreover, they are all exactly FASTOP_SIZE bytes long, so functions for
- * different operand sizes can be reached by calculation, rather than a jump
- * table (which would be bigger than the code).
- *
- * fastop functions are declared as taking a never-defined fastop parameter,
- * so they can't be called from C directly.
- */
-
-struct fastop;
-
 struct opcode {
 	u64 flags : 56;
 	u64 intercept : 8;
@@ -311,8 +292,19 @@ static void invalidate_registers(struct x86_emulate_ctxt *ctxt)
 #define ON64(x)
 #endif
 
-typedef void (*fastop_t)(struct fastop *);
-
+/*
+ * fastop functions have a special calling convention:
+ *
+ * dst:    rax        (in/out)
+ * src:    rdx        (in/out)
+ * src2:   rcx        (in)
+ * flags:  rflags     (in/out)
+ * ex:     rsi        (in:fastop pointer, out:zero if exception)
+ *
+ * Moreover, they are all exactly FASTOP_SIZE bytes long, so functions for
+ * different operand sizes can be reached by calculation, rather than a jump
+ * table (which would be bigger than the code).
+ */
 static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
 
 #define __FOP_FUNC(name) \
@@ -5683,7 +5675,7 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
 
 	if (ctxt->execute) {
 		if (ctxt->d & Fastop)
-			rc = fastop(ctxt, (fastop_t)ctxt->execute);
+			rc = fastop(ctxt, ctxt->fop);
 		else
 			rc = ctxt->execute(ctxt);
 		if (rc != X86EMUL_CONTINUE)