Re: [PATCH 4/6] accel/tcg: Declare missing cpu_loop_exit*() stubs

[Claudio Fontana <cfontana@suse.de>]

On 1/17/21 5:48 PM, Philippe Mathieu-Daudé wrote:
> cpu_loop_exit*() functions are declared in accel/tcg/cpu-exec-common.c,
> and are not available when TCG accelerator is not built. Add stubs so
> linking without TCG succeed.
> 
> Problematic files:
> 
> - hw/semihosting/console.c in qemu_semihosting_console_inc()
> - hw/ppc/spapr_hcall.c in h_confer()
> - hw/s390x/ipl.c in s390_ipl_reset_request()
> - hw/misc/mips_itu.c
> 
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> ---
> I suppose the s390x kvm-only build didn't catch this because
> of compiler optimization:
> 
> in s390_ipl_reset_request():
> 
> 640     if (tcg_enabled()) {
> 641         cpu_loop_exit(cs);
> 642     }

Ciao Philippe,

yes I have seen this a lot on x86 also, and seems to depend on the compiler used.

On OpenSUSE 15.2, with gcc based on 7.5.0, I am getting this optimization also, and so no error happens in these cases.

It is a bit inconvenient because I have to rely completely on the CI to catch these situations.

Ciao,

Claudio



> 
> and "sysemu/tcg.h" is:
> 
>  13 #ifdef CONFIG_TCG
>  14 extern bool tcg_allowed;
>  15 #define tcg_enabled() (tcg_allowed)
>  16 #else
>  17 #define tcg_enabled() 0
>  18 #endif
> ---
>  accel/stubs/tcg-stub.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/accel/stubs/tcg-stub.c b/accel/stubs/tcg-stub.c
> index 8c18d3eabdd..2304606f8e0 100644
> --- a/accel/stubs/tcg-stub.c
> +++ b/accel/stubs/tcg-stub.c
> @@ -28,3 +28,13 @@ void *probe_access(CPUArchState *env, target_ulong addr, int size,
>       /* Handled by hardware accelerator. */
>       g_assert_not_reached();
>  }
> +
> +void QEMU_NORETURN cpu_loop_exit(CPUState *cpu)
> +{
> +    g_assert_not_reached();
> +}
> +
> +void QEMU_NORETURN cpu_loop_exit_restore(CPUState *cpu, uintptr_t pc)
> +{
> +    g_assert_not_reached();
> +}
>