cpu: Avoid QOM casts for CPU()

CPU address spaces touching load and store helpers as well as the
movement of (almost) all fields from CPU_COMMON to CPUState have led to
a noticeable increase of CPU() usage in "hot" paths for both TCG and KVM.

While CPU()'s OBJECT_CHECK() might help detect development errors, i.e.
in form of crashes due to QOM vs. non-QOM mismatches rather than QOM
type mismatches, it is not really needed at runtime since mostly used in
CPU-specific paths, coming from a target-specific CPU subtype. If that
pointer is damaged, other errors are highly likely to occur elsewhere
anyway.

Keep the CPU() macro for a consistent developer experience and for
flexibility to exchange its implementation, but turn it into a pure,
unchecked C cast for now.

Compare commit 6e42be7.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>

include/qom/cpu.h

@@ -53,7 +53,12 @@ typedef uint64_t vaddr;
 
 #define TYPE_CPU "cpu"
 
-#define CPU(obj) OBJECT_CHECK(CPUState, (obj), TYPE_CPU)
+/* Since this macro is used a lot in hot code paths and in conjunction with
+ * FooCPU *foo_env_get_cpu(), we deviate from usual QOM practice by using
+ * an unchecked cast.
+ */
+#define CPU(obj) ((CPUState *)(obj))
+
 #define CPU_CLASS(class) OBJECT_CLASS_CHECK(CPUClass, (class), TYPE_CPU)
 #define CPU_GET_CLASS(obj) OBJECT_GET_CLASS(CPUClass, (obj), TYPE_CPU)