Re: [RFC v12 31/65] target/arm: move exception code out of tcg/helper.c

[Richard Henderson]

On 4/8/21 3:56 AM, Claudio Fontana wrote:
> It is a struggle to balance making TCG-only symbols "invisible" to kvm-builds, 
> so one developing for KVM only can forget about them,
> and keeping TCG things contained,
>
> and on the other side opening up the ability to use tcg_enabled(), which 
> requires those prototypes to be visible.
>
> Where do you see that balance?

The short answer is I'd keep the prototypes available, always.

The longer answer is that I'd make more use of QEMU_ERROR().

I think I described this off-hand before, but let me develop this thought more 
completely.  Let's put some defines in a handy common header file:

#ifdef CONFIG_TCG
#define REQUIRE_TCG
#else
#define REQUIRE_TCG  QEMU_ERROR("tcg function is reachable")
#endif

and so on for all of the other accelerators, as well as

#ifdef CONFIG_USER_ONLY
#define REQUIRE_USER_ONLY
#define REQUIRE_SYSEMU QEMU_ERROR("sysemu function is reachable")
#else
#define REQUIRE_USER_ONLY \
  QEMU_ERROR("user-only function is reachable")
#define REQUIRE_SYSEMU
#endif

Then, in target/foo/bar.h, instead of

#ifdef CONFIG_TCG
void some_function(...);
#endif

we write

void some_function(...) REQUIRE_TCG;

which is self-documentary, and also generates a compile error when the function 
call has not been eliminated by a guard such as tcg_enabled (as opposed to 
waiting until link time for the missing symbol).


r~